Table of Contents
This chapter describes the syntax for the SQL statements supported in MySQL.
DELETE [LOW_PRIORITY] [QUICK] [IGNORE] FROM tbl_name
[WHERE where_definition]
[ORDER BY ...]
[LIMIT row_count]
Multiple-table syntax:
DELETE [LOW_PRIORITY] [QUICK] [IGNORE]
tbl_name[.*] [, tbl_name[.*] ...]
FROM table_references
[WHERE where_definition]
Or:
DELETE [LOW_PRIORITY] [QUICK] [IGNORE]
FROM tbl_name[.*] [, tbl_name[.*] ...]
USING table_references
[WHERE where_definition]
DELETE deletes rows from tbl_name that satisfy the condition given by where_definition, and returns the number of records deleted.
If you issue a DELETE statement with no WHERE clause, all rows are deleted. A faster way to do this, when you don't want to know the number of deleted rows, is to use TRUNCATE TABLE. See TRUNCATE. In MySQL 3.23, DELETE without a WHERE clause returns zero as the number of affected records.
In MySQL 3.23, if you really want to know how many records are deleted when you are deleting all rows, and are willing to suffer a speed penalty, you can use a DELETE statement that includes a WHERE clause with an expression that is true for every row. For example:
mysql> DELETE FROM tbl_name WHERE 1>0;
This is much slower than TRUNCATE tbl_name, because it deletes rows one at a time.
If you delete the row containing the maximum value for an AUTO_INCREMENT column, the value will be reused for an ISAM or BDB table, but not for a MyISAM or InnoDB table. If you delete all rows in the table with DELETE FROM tbl_name (without a WHERE) in AUTOCOMMIT mode, the sequence starts over for all table types except for InnoDB and (as of MySQL 4.0) MyISAM. There are some exceptions to this behavior for InnoDB tables, discussed in InnoDB auto-increment column.
For MyISAM and BDB tables, you can specify an AUTO_INCREMENT secondary column in a multiple-column key. In this case, reuse of values deleted from the top of the sequence occurs even for MyISAM tables. See example-AUTO_INCREMENT.
The DELETE statement supports the following modifiers:
If you specify the LOW_PRIORITY keyword, execution of the DELETE is delayed until no other clients are reading from the table.
For MyISAM tables, if you specify the QUICK keyword, the storage engine does not merge index leaves during delete, which may speed up certain kind of deletes.
The IGNORE keyword causes MySQL to ignore all errors during the process of deleting rows. (Errors encountered during the parsing stage are processed in the usual manner.) Errors that are ignored due to the use of this option are returned as warnings. This option first appeared in MySQL 4.1.1.
The speed of delete operations may also be affected by factors discussed in the section called “Speed of DELETE Statements”.
In MyISAM tables, deleted records are maintained in a linked list and subsequent INSERT operations reuse old record positions. To reclaim unused space and reduce file sizes, use the OPTIMIZE TABLE statement or the myisamchk utility to reorganize tables. OPTIMIZE TABLE is easier, but myisamchk is faster. See OPTIMIZE TABLE and the section called “Table Optimization”.
The MySQL-specific LIMIT row_count option to DELETE tells the server the maximum number of rows to be deleted before control is returned to the client. This can be used to ensure that a specific DELETE statement doesn't take too much time. You can simply repeat the DELETE statement until the number of affected rows is less than the LIMIT value.
If the DELETE statement includes an ORDER BY clause, the rows are deleted in the order specified by the clause. This is really useful only in conjunction with LIMIT. For example, the following statement finds rows matching the WHERE clause, sorts them in timestamp order, and deletes the first (oldest) one:
DELETE FROM somelog WHERE user = 'jcole' ORDER BY timestamp LIMIT 1
ORDER BY can be used with DELETE beginning with MySQL 4.0.0.
From MySQL 4.0, you can specify multiple tables in the DELETE statement to delete rows from one or more tables depending on a particular condition in multiple tables. However, you cannot use ORDER BY or LIMIT in a multiple-table DELETE.
The first multiple-table DELETE syntax is supported starting from MySQL 4.0.0. The second is supported starting from MySQL 4.0.2. The table_references part lists the tables involved in the join. Its syntax is described in JOIN.
For the first syntax, only matching rows from the tables listed before the FROM clause are deleted. For the second syntax, only matching rows from the tables listed in the FROM clause (before the USING clause) are deleted. The effect is that you can delete rows from many tables at the same time and also have additional tables that are used for searching:
DELETE t1, t2 FROM t1, t2, t3 WHERE t1.id=t2.id AND t2.id=t3.id;
Or:
DELETE FROM t1, t2 USING t1, t2, t3 WHERE t1.id=t2.id AND t2.id=t3.id;
These statements use all three files when searching for rows to delete, but delete matching rows only from tables t1 and t2.
The examples show inner joins using the comma operator, but multiple-table DELETE statements can use any type of join allowed in SELECT statements, such as LEFT JOIN.
The syntax allows .* after the table names for compatibility with Access.
If you use a multiple-table DELETE statement involving InnoDB tables for which there are foreign key constraints, the MySQL optimizer might process tables in an order that differs from that of their parent/child relationship. In this case, the statement fails and rolls back. Instead, delete from a single table and rely on the ON DELETE capabilities that InnoDB provides to cause the other tables to be modified accordingly.
Note: In MySQL 4.0, you should refer to the table names to be deleted with the true table name. In MySQL 4.1, you must use the alias (if one was given) when referring to a table name:
In MySQL 4.0:
DELETE test FROM test AS t1, test2 WHERE ...
In MySQL 4.1:
DELETE t1 FROM test AS t1, test2 WHERE ...
The reason we didn't make this change in 4.0 is that we didn't want to break any old 4.0 applications that were using the old syntax.
Currently, you cannot delete from a table and select from the same table in a subquery.
DO expr [, expr] ...
DO executes the expressions but doesn't return any results. This is shorthand for SELECT expr, ..., but has the advantage that it's slightly faster when you don't care about the result.
DO is useful mainly with functions that have side effects, such as RELEASE_LOCK(). DO was added in MySQL 3.23.47.
HANDLER tbl_name OPEN [ AS alias ]
HANDLER tbl_name READ index_name { = | >= | <= | < } (value1,value2,...)
[ WHERE where_condition ] [LIMIT ... ]
HANDLER tbl_name READ index_name { FIRST | NEXT | PREV | LAST }
[ WHERE where_condition ] [LIMIT ... ]
HANDLER tbl_name READ { FIRST | NEXT }
[ WHERE where_condition ] [LIMIT ... ]
HANDLER tbl_name CLOSE
The HANDLER statement provides direct access to table storage engine interfaces. It is available for MyISAM tables as MySQL 4.0.0 and InnoDB tables as of MySQL 4.0.3. The HANDLER ... OPEN statement opens a table, making it accessible via subsequent HANDLER ... READ statements. This table object is not shared by other threads and is not closed until the thread calls HANDLER ... CLOSE or the thread terminates. If you open the table using an alias, further references to the table with other HANDLER statements must use the alias rather than the table name.
The first HANDLER ... READ syntax fetches a row where the index specified satisfies the given values and the WHERE condition is met. If you have a multiple-column index, specify the index column values as a comma-separated list. Either specify values for all the columns in the index, or specify values for a leftmost prefix of the index columns. Suppose that an index includes three columns named col_a, col_b, and col_c, in that order. The HANDLER statement can specify values for all three columns in the index, or for the columns in a leftmost prefix. For example:
HANDLER ... index_name = (col_a_val,col_b_val,col_c_val) ... HANDLER ... index_name = (col_a_val,col_b_val) ... HANDLER ... index_name = (col_a_val) ...
The second HANDLER ... READ syntax fetches a row from the table in index order that that matches WHERE condition.
The third HANDLER ... READ syntax fetches a row from the table in natural row order that matches the WHERE condition. It is faster than HANDLER tbl_name READ index_name when a full table scan is desired. Natural row order is the order in which rows are stored in a MyISAM table data file. This statement works for InnoDB tables as well, but there is no such concept because there is no separate data file.
Without a LIMIT clause, all forms of HANDLER ... READ fetch a single row if one is available. To return a specific number of rows, include a LIMIT clause. It has the same syntax as for the SELECT statement. See SELECT.
HANDLER ... CLOSE closes a table that was opened with HANDLER ... OPEN.
Note: To use the HANDLER interface to refer to a table's PRIMARY KEY, use the quoted identifier `PRIMARY`:
HANDLER tbl_name READ `PRIMARY` > (...);
HANDLER is a somewhat low-level statement. For example, it does not provide consistency. That is, HANDLER ... OPEN does not take a snapshot of the table, and does not lock the table. This means that after a HANDLER ... OPEN statement is issued, table data can be modified (by this or any other thread) and these modifications might appear only partially in HANDLER ... NEXT or HANDLER ... PREV scans.
There are several reasons to use the HANDLER interface instead of normal SELECT statements:
HANDLER is faster than SELECT:
A designated storage engine handler object is allocated for the HANDLER ... OPEN. The object is reused for the following HANDLER statements for the table; it need not be reinitialized for each one.
There is less parsing involved.
There is no optimizer or query-checking overhead.
The table doesn't have to be locked between two handler requests.
The handler interface doesn't have to provide a consistent look of the data (for example, dirty reads are allowed), so the storage engine can use optimizations that SELECT doesn't normally allow.
HANDLER makes it much easier to port applications that use an ISAM-like interface to MySQL.
HANDLER allows you to traverse a database in a manner that is not easy (or perhaps even impossible) to do with SELECT. The HANDLER interface is a more natural way to look at data when working with applications that provide an interactive user interface to the database.
INSERT [LOW_PRIORITY | DELAYED | HIGH_PRIORITY] [IGNORE]
[INTO] tbl_name [(col_name,...)]
VALUES ({expr | DEFAULT},...),(...),...
[ ON DUPLICATE KEY UPDATE col_name=expr, ... ]
Or:
INSERT [LOW_PRIORITY | DELAYED | HIGH_PRIORITY] [IGNORE]
[INTO] tbl_name
SET col_name={expr | DEFAULT}, ...
[ ON DUPLICATE KEY UPDATE col_name=expr, ... ]
Or:
INSERT [LOW_PRIORITY | DELAYED | HIGH_PRIORITY] [IGNORE]
[INTO] tbl_name [(col_name,...)]
SELECT ...
INSERT inserts new rows into an existing table. The INSERT ... VALUES and INSERT ... SET forms of the statement insert rows based on explicitly specified values. The INSERT ... SELECT form inserts rows selected from another table or tables. The INSERT ... VALUES form with multiple value lists is supported in MySQL 3.22.5 or later. The INSERT ... SET syntax is supported in MySQL 3.22.10 or later. INSERT ... SELECT is discussed further in See INSERT SELECT. tbl_name is the table into which rows should be inserted. The columns for which the statement provides values can be specified as follows:
The column name list or the SET clause indicates the columns explicitly.
If you do not specify the column list for INSERT ... VALUES or INSERT ... SELECT, values for every column in the table must be provided in the VALUES() list or by the SELECT. If you don't know the order of the columns in the table, use DESCRIBE tbl_name to find out.
Column values can be given in several ways:
If you are not running in strict mode, any column not explicitly given a value is set to its default (explicit or implicit) value. For example, if you specify a column list that doesn't name all the columns in the table, unnamed columns are set to their default values. Default value assignment is described in CREATE TABLE. See the section called “Constraint NOT NULL and DEFAULT Values”.
If you want INSERT statements to generate an error unless you explicitly specify values for all columns that don't have a default value, you should use STRICT mode. See the section called “The Server SQL Mode”.
You can use the keyword DEFAULT to explicitly set a column to its default value. (New in MySQL 4.0.3.) This makes it easier to write INSERT statements that assign values to all but a few columns, because it allows you to avoid writing an incomplete VALUES list that does not include a value for each column in the table. Otherwise, you would have to write out the list of column names corresponding to each value in the VALUES list.
As of MySQL 4.1.0, you can use DEFAULT(col_name) as a more general form that can be used in expressions to produce a column's default value.
If both the column list and the VALUES list are empty, INSERT creates a row with each column set to its default value:
mysql> INSERT INTO tbl_name () VALUES();
You can specify an expression expr to provide a column value. This might involve type conversion if the type of the expression does not match the type of the column, and conversion of a given value can result in different inserted values depending on the column type. For example, inserting the string '1999.0e-2' into an INT, FLOAT, DECIMAL(10,6), or YEAR column results in the values 1999, 19.9921, 19.992100, and 1999. The reason the value stored in the INT and YEAR columns is 1999 is that the string-to-integer conversion looks only at as much of the initial part of the string as may be considered a valid integer or year. For the floating-point and fixed-point columns, the string-to-floating-point conversion considers the entire string as a valid floating-point value.
An expression expr can refer to any column that was set earlier in a value list. For example, you can do this because the value for col2 refers to col1, which has already been assigned:
mysql> INSERT INTO tbl_name (col1,col2) VALUES(15,col1*2);
But you cannot do this because the value for col1 refers to col2, which is assigned after col1:
mysql> INSERT INTO tbl_name (col1,col2) VALUES(col2*2,15);
One exception involves columns that contain AUTO_INCREMENT values. Because the AUTO_INCREMENT value is generated after other value assignments, any reference to an AUTO_INCREMENT column in the assignment will return a 0.
The INSERT statement supports the following modifiers:
If you specify the DELAYED keyword, the server puts the row or rows to be inserted into a buffer, and the client issuing the INSERT DELAYED statement then can continue on. If the table is busy, the server holds the rows. When the table becomes free, it begins inserting rows, checking periodically to see whether there are new read requests for the table. If there are, the delayed row queue is suspended until the table becomes free again. See INSERT DELAYED. DELAYED was added in MySQL 3.22.5.
If you specify the LOW_PRIORITY keyword, execution of the INSERT is delayed until no other clients are reading from the table. This includes other clients that began reading while existing clients are reading, and while the INSERT LOW_PRIORITY statement is waiting. It is possible, therefore, for a client that issues an INSERT LOW_PRIORITY statement to wait for a very long time (or even forever) in a read-heavy environment. (This is in contrast to INSERT DELAYED, which lets the client continue at once.) See INSERT DELAYED. Note that LOW_PRIORITY should normally not be used with MyISAM tables because doing so disables concurrent inserts. See MyISAM. LOW_PRIORITY was added in MySQL 3.22.5.
If you specify the HIGH_PRIORITY keyword, it overrides the effect of the --low-priority-updates option if the server was started with that option. It also causes concurrent inserts not to be used. HIGH_PRIORITY was added in MySQL 3.23.11.
The rows-affected value for an INSERT can be obtained using the mysql_affected_rows() C API function. See mysql_affected_rows().
If you specify the IGNORE keyword in an INSERT with many rows, any rows that duplicate an existing UNIQUE index or PRIMARY KEY value in the table are ignored and are not inserted. If you do not specify IGNORE, the insert is aborted if there is any row that duplicates an existing key value. You can determine with the mysql_info() C API function how many rows were inserted into the table.
If you specify the ON DUPLICATE KEY UPDATE clause (new in MySQL 4.1.0), and a row is inserted that would cause a duplicate value in a UNIQUE index or PRIMARY KEY, an UPDATE of the old row is performed. For example, if column a is declared as UNIQUE and already contains the value 1, the following two statements have identical effect:
mysql> INSERT INTO table (a,b,c) VALUES (1,2,3)
-> ON DUPLICATE KEY UPDATE c=c+1;
mysql> UPDATE table SET c=c+1 WHERE a=1;
The rows-affected value is 1 if the row is inserted as a new record and 2 if an existing record is updated.
Note: If column b is unique too, the INSERT would be equivalent to this UPDATE statement instead:
mysql> UPDATE table SET c=c+1 WHERE a=1 OR b=2 LIMIT 1;
If a=1 OR b=2 matches several rows, only one row is updated! In general, you should try to avoid using the ON DUPLICATE KEY clause on tables with multiple UNIQUE keys.
As of MySQL 4.1.1, you can use the VALUES(col_name) function in the UPDATE clause to refer to column values from the INSERT part of the INSERT ... UPDATE statement. In other words, VALUES(col_name) in the UPDATE clause refers to the value of col_name that would be inserted if no duplicate-key conflict occurred. This function is especially useful in multiple-row inserts. The VALUES() function is meaningful only in INSERT ... UPDATE statements and returns NULL otherwise.
Example:
mysql> INSERT INTO table (a,b,c) VALUES (1,2,3),(4,5,6)
-> ON DUPLICATE KEY UPDATE c=VALUES(a)+VALUES(b);
That statement is identical to the following two statements:
mysql> INSERT INTO table (a,b,c) VALUES (1,2,3)
-> ON DUPLICATE KEY UPDATE c=3;
mysql> INSERT INTO table (a,b,c) VALUES (4,5,6)
-> ON DUPLICATE KEY UPDATE c=9;
When you use ON DUPLICATE KEY UPDATE, the DELAYED option is ignored.
You can find the value used for an AUTO_INCREMENT column by using the LAST_INSERT_ID() function. From within the C API, use the mysql_insert_id() function. However, note that the two functions do not behave quite identically under all circumstances. The behavior of INSERT statements with respect to AUTO_INCREMENT columns is discussed further in the section called “Information Functions” and mysql_insert_id().
If you use an INSERT ... VALUES statement with multiple value lists or INSERT ... SELECT, the statement returns an information string in this format:
Records: 100 Duplicates: 0 Warnings: 0
Records indicates the number of rows processed by the statement. (This is not necessarily the number of rows actually inserted. Duplicates can be non-zero.) Duplicates indicates the number of rows that couldn't be inserted because they would duplicate some existing unique index value. Warnings indicates the number of attempts to insert column values that were problematic in some way. Warnings can occur under any of the following conditions:
Inserting NULL into a column that has been declared NOT NULL. For multiple-row INSERT statements or INSERT ... SELECT statements, the column is set to the default value appropriate for the column type. This is 0 for numeric types, the empty string ('') for string types, and the “zero” value for date and time types.
Setting a numeric column to a value that lies outside the column's range. The value is clipped to the closest endpoint of the range.
Assigning a value such as '10.34 a' to a numeric column. The trailing non-numeric text is stripped off and the remaining numeric part is inserted. If the string value has no leading numeric part, the column is set to 0.
Inserting a string into a string column (CHAR, VARCHAR, TEXT, or BLOB) that exceeds the column's maximum length. The value is truncated to the column's maximum length.
Inserting a value into a date or time column that is illegal for the column type. The column is set to the appropriate zero value for the type.
If you are using the C API, the information string can be obtained by invoking the mysql_info() function. See mysql_info().
INSERT [LOW_PRIORITY] [IGNORE] [INTO] tbl_name [(column_list)]
SELECT ...
With INSERT ... SELECT, you can quickly insert many rows into a table from one or many tables. For example:
INSERT INTO tbl_temp2 (fld_id)
SELECT tbl_temp1.fld_order_id
FROM tbl_temp1 WHERE tbl_temp1.fld_order_id > 100;
The following conditions hold for an INSERT ... SELECT statement:
Prior to MySQL 4.0.1, INSERT ... SELECT implicitly operates in IGNORE mode. As of MySQL 4.0.1, specify IGNORE explicitly to ignore records that would cause duplicate-key violations.
Do not use DELAYED with INSERT ... SELECT.
Prior to MySQL 4.0.14, the target table of the INSERT statement cannot appear in the FROM clause of the SELECT part of the query. This limitation is lifted in 4.0.14.
AUTO_INCREMENT columns work as usual.
To ensure that the binary log can be used to re-create the original tables, MySQL will not allow concurrent inserts during INSERT ... SELECT.
Currently, you cannot insert into a table and select from the same table in a subquery.
You can use REPLACE instead of INSERT to overwrite old rows. REPLACE is the counterpart to INSERT IGNORE in the treatment of new rows that contain unique key values that duplicate old rows: The new rows are used to replace the old rows rather than being discarded.
INSERT DELAYED ...
The DELAYED option for the INSERT statement is a MySQL extension to standard SQL that is very useful if you have clients that can't wait for the INSERT to complete. This is a common problem when you use MySQL for logging and you also periodically run SELECT and UPDATE statements that take a long time to complete. DELAYED was introduced in MySQL 3.22.15.
When a client uses INSERT DELAYED, it gets an okay from the server at once, and the row is queued to be inserted when the table is not in use by any other thread.
Another major benefit of using INSERT DELAYED is that inserts from many clients are bundled together and written in one block. This is much faster than doing many separate inserts.
There are some constraints on the use of DELAYED:
INSERT DELAYED works only with MyISAM and ISAM tables. As of MySQL 4.0.14, it also works with InnoDB tables. For MyISAM tables, if there are no free blocks in the middle of the data file, concurrent SELECT and INSERT statements are supported. Under these circumstances, you very seldom need to use INSERT DELAYED with MyISAM. See the section called “The MyISAM Storage Engine”.
INSERT DELAYED should be used only for INSERT statements that specify value lists. This is enforced as of MySQL 4.0.18. The server ignores DELAYED for INSERT DELAYED ... SELECT statements.
The server ignores DELAYED for INSERT DELAYED ... ON DUPLICATE UPDATE statements.
Because the statement returns immediately before the rows are inserted, you cannot use LAST_INSERT_ID() to get the AUTO_INCREMENT value the statement might generate.
DELAYED rows are not visible to SELECT statements until they actually have been inserted.
Note that currently the queued rows are held only in memory until they are inserted into the table. This means that if you terminate mysqld forcibly (for example, with kill -9) or if mysqld dies unexpectedly, any queued rows that have not been written to disk are lost!
The following describes in detail what happens when you use the DELAYED option to INSERT or REPLACE. In this description, the “thread” is the thread that received an INSERT DELAYED statement and “handler” is the thread that handles all INSERT DELAYED statements for a particular table.
When a thread executes a DELAYED statement for a table, a handler thread is created to process all DELAYED statements for the table, if no such handler already exists.
The thread checks whether the handler has acquired a DELAYED lock already; if not, it tells the handler thread to do so. The DELAYED lock can be obtained even if other threads have a READ or WRITE lock on the table. However, the handler will wait for all ALTER TABLE locks or FLUSH TABLES to ensure that the table structure is up to date.
The thread executes the INSERT statement, but instead of writing the row to the table, it puts a copy of the final row into a queue that is managed by the handler thread. Any syntax errors are noticed by the thread and reported to the client program.
The client cannot obtain from the server the number of duplicate records or the AUTO_INCREMENT value for the resulting row, because the INSERT returns before the insert operation has been completed. (If you use the C API, the mysql_info() function doesn't return anything meaningful, for the same reason.)
The binary log is updated by the handler thread when the row is inserted into the table. In case of multiple-row inserts, the binary log is updated when the first row is inserted.
After every delayed_insert_limit rows are written, the handler checks whether any SELECT statements are still pending. If so, it allows these to execute before continuing.
When the handler has no more rows in its queue, the table is unlocked. If no new INSERT DELAYED statements are received within delayed_insert_timeout seconds, the handler terminates.
If more than delayed_queue_size rows are pending already in a specific handler queue, the thread requesting INSERT DELAYED waits until there is room in the queue. This is done to ensure that the mysqld server doesn't use all memory for the delayed memory queue.
The handler thread shows up in the MySQL process list with delayed_insert in the Command column. It will be killed if you execute a FLUSH TABLES statement or kill it with KILL thread_id. However, before exiting, it will first store all queued rows into the table. During this time it will not accept any new INSERT statements from another thread. If you execute an INSERT DELAYED statement after this, a new handler thread will be created.
Note that this means that INSERT DELAYED statements have higher priority than normal INSERT statements if there is an INSERT DELAYED handler already running! Other update statements will have to wait until the INSERT DELAYED queue is empty, someone terminates the handler thread (with KILL thread_id), or someone executes FLUSH TABLES.
The following status variables provide information about INSERT DELAYED statements:
| Status Variable | Meaning |
| Delayed_insert_threads | Number of handler threads |
| Delayed_writes | Number of rows written with INSERT DELAYED |
| Not_flushed_delayed_rows | Number of rows waiting to be written |
You can view these variables by issuing a SHOW STATUS statement or by executing a mysqladmin extended-status command.
Note that INSERT DELAYED is slower than a normal INSERT if the table is not in use. There is also the additional overhead for the server to handle a separate thread for each table for which there are delayed rows. This means that you should use INSERT DELAYED only when you are really sure that you need it!
LOAD DATA [LOW_PRIORITY | CONCURRENT] [LOCAL] INFILE 'file_name.txt'
[REPLACE | IGNORE]
INTO TABLE tbl_name
[FIELDS
[TERMINATED BY '\t']
[[OPTIONALLY] ENCLOSED BY '']
[ESCAPED BY '\\' ]
]
[LINES
[STARTING BY '']
[TERMINATED BY '\n']
]
[IGNORE number LINES]
[(col_name,...)]
The LOAD DATA INFILE statement reads rows from a text file into a table at a very high speed. For more information about the efficiency of INSERT versus LOAD DATA INFILE and speeding up LOAD DATA INFILE, the section called “Speed of INSERT Statements”.
You can also load data files by using the mysqlimport utility; it operates by sending a LOAD DATA INFILE statement to the server. The --local option causes mysqlimport to read data files from the client host. You can specify the --compress option to get better performance over slow networks if the client and server support the compressed protocol. See mysqlimport.
If you specify the LOW_PRIORITY keyword, execution of the LOAD DATA statement is delayed until no other clients are reading from the table.
If you specify the CONCURRENT keyword with a MyISAM table that satisfies the condition for concurrent inserts (that is, it contains no free blocks in the middle), then other threads can retrieve data from the table while LOAD DATA is executing. Using this option affects the performance of LOAD DATA a bit, even if no other thread is using the table at the same time.
If the LOCAL keyword is specified, it is interpreted with respect to the client end of the connection:
If LOCAL is specified, the file is read by the client program on the client host and sent to the server.
If LOCAL is not specified, the file must be located on the server host and is read directly by the server.
LOCAL is available in MySQL 3.22.6 or later.
For security reasons, when reading text files located on the server, the files must either reside in the database directory or be readable by all. Also, to use LOAD DATA INFILE on server files, you must have the FILE privilege. See the section called “Privileges Provided by MySQL”.
Using LOCAL is a bit slower than letting the server access the files directly, because the contents of the file must be sent over the connection by the client to the server. On the other hand, you do not need the FILE privilege to load local files.
As of MySQL 3.23.49 and MySQL 4.0.2 (4.0.13 on Windows), LOCAL works only if your server and your client both have been enabled to allow it. For example, if mysqld was started with --local-infile=0, LOCAL will not work. See LOAD DATA LOCAL.
If you need LOAD DATA to read from a pipe, you can use the following technique (here we load the listing of the '/' directory into a table):
mkfifo /mysql/db/x/x chmod 666 /mysql/db/x/x find / -ls > /mysql/db/x/x mysql -e "LOAD DATA INFILE 'x' INTO TABLE x" x
If you are using a version of MySQL older than 3.23.25, you can use this technique only with LOAD DATA LOCAL INFILE.
If you are using MySQL before Version 3.23.24, you can't read from a FIFO with LOAD DATA INFILE. If you need to read from a FIFO (for example, the output from gunzip), use LOAD DATA LOCAL INFILE instead.
When locating files on the server host, the server uses the following rules:
If an absolute pathname is given, the server uses the pathname as is.
If a relative pathname with one or more leading components is given, the server searches for the file relative to the server's data directory.
If a filename with no leading components is given, the server looks for the file in the database directory of the default database.
Note that these rules mean that a file named as ./myfile.txt is read from the server's data directory, whereas the same file named as myfile.txt is read from the database directory of the default database. For example, the following LOAD DATA statement reads the file data.txt from the database directory for db1 because db1 is the current database, even though the statement explicitly loads the file into a table in the db2 database:
mysql> USE db1; mysql> LOAD DATA INFILE 'data.txt' INTO TABLE db2.my_table;
The REPLACE and IGNORE keywords control handling of input records that duplicate existing records on unique key values.
If you specify REPLACE, input rows replace existing rows (in other words, rows that have the same value for a primary or unique index as an existing row). See REPLACE.
If you specify IGNORE, input rows that duplicate an existing row on a unique key value are skipped. If you don't specify either option, the behavior depends on whether or not the LOCAL keyword is specified. Without LOCAL, an error occurs when a duplicate key value is found, and the rest of the text file is ignored. With LOCAL, the default behavior is the same as if IGNORE is specified; this is because the server has no way to stop transmission of the file in the middle of the operation.
If you want to ignore foreign key constraints during the load operation, you can issue a SET FOREIGN_KEY_CHECKS=0 statement before executing LOAD DATA.
If you use LOAD DATA INFILE on an empty MyISAM table, all non-unique indexes are created in a separate batch (as for REPAIR TABLE). This normally makes LOAD DATA INFILE much faster when you have many indexes. Normally this is very fast, but in some extreme cases, you can create the indexes even faster by turning them off with ALTER TABLE .. DISABLE KEYS before loading the file into the table and using ALTER TABLE .. ENABLE KEYS to re-create the indexes after loading the file. See the section called “Speed of INSERT Statements”.
LOAD DATA INFILE is the complement of SELECT ... INTO OUTFILE. See SELECT. To write data from a table to a file, use SELECT ... INTO OUTFILE. To read the file back into a table, use LOAD DATA INFILE. The syntax of the FIELDS and LINES clauses is the same for both statements. Both clauses are optional, but FIELDS must precede LINES if both are specified.
If you specify a FIELDS clause, each of its subclauses (TERMINATED BY, [OPTIONALLY] ENCLOSED BY, and ESCAPED BY) is also optional, except that you must specify at least one of them.
If you don't specify a FIELDS clause, the defaults are the same as if you had written this:
FIELDS TERMINATED BY '\t' ENCLOSED BY '' ESCAPED BY '\\'
If you don't specify a LINES clause, the default is the same as if you had written this:
LINES TERMINATED BY '\n' STARTING BY ''
In other words, the defaults cause LOAD DATA INFILE to act as follows when reading input:
Look for line boundaries at newlines.
Do not skip over any line prefix.
Break lines into fields at tabs.
Do not expect fields to be enclosed within any quoting characters.
Interpret occurrences of tab, newline, or ‘\’ preceded by ‘\’ as literal characters that are part of field values.
Conversely, the defaults cause SELECT ... INTO OUTFILE to act as follows when writing output:
Write tabs between fields.
Do not enclose fields within any quoting characters.
Use ‘\’ to escape instances of tab, newline, or ‘\’ that occur within field values.
Write newlines at the ends of lines.
Note that to write FIELDS ESCAPED BY '\\', you must specify two backslashes for the value to be read as a single backslash.
Note: If you have generated the text file on a Windows system, you might have to use LINES TERMINATED BY '\r\n' to read the file properly, because Windows programs typically use two characters as a line terminator. Some programs, such as WordPad, might use \r as a line terminator when writing files. To read such files, use LINES TERMINATED BY '\r'.
If all the lines you want to read in have a common prefix that you want to ignore, you can use LINES STARTING BY 'prefix_string' to skip over the prefix (and anything before it). If a line doesn't include the prefix, the entire line is skipped. Note that prefix_string may be in the middle of the line!
Example:
mysql> LOAD DATA INFILE '/tmp/test.txt'
-> INTO TABLE test LINES STARTING BY "xxx";
With this you can read in a file that contains something like:
xxx"Row",1 something xxx"Row",2
And just get the data ("row",1) and ("row",2).
The IGNORE number LINES option can be used to ignore lines at the start of the file. For example, you can use IGNORE 1 LINES to skip over an initial header line containing column names:
mysql> LOAD DATA INFILE '/tmp/test.txt'
-> INTO TABLE test IGNORE 1 LINES;
When you use SELECT ... INTO OUTFILE in tandem with LOAD DATA INFILE to write data from a database into a file and then read the file back into the database later, the field- and line-handling options for both statements must match. Otherwise, LOAD DATA INFILE will not interpret the contents of the file properly. Suppose that you use SELECT ... INTO OUTFILE to write a file with fields delimited by commas:
mysql> SELECT * INTO OUTFILE 'data.txt'
-> FIELDS TERMINATED BY ','
-> FROM table2;
To read the comma-delimited file back in, the correct statement would be:
mysql> LOAD DATA INFILE 'data.txt' INTO TABLE table2
-> FIELDS TERMINATED BY ',';
If instead you tried to read in the file with the statement shown here, it wouldn't work because it instructs LOAD DATA INFILE to look for tabs between fields:
mysql> LOAD DATA INFILE 'data.txt' INTO TABLE table2
-> FIELDS TERMINATED BY '\t';
The likely result is that each input line would be interpreted as a single field.
LOAD DATA INFILE can be used to read files obtained from external sources, too. For example, a file in dBASE format will have fields separated by commas and enclosed within double quotes. If lines in the file are terminated by newlines, the statement shown here illustrates the field- and line-handling options you would use to load the file:
mysql> LOAD DATA INFILE 'data.txt' INTO TABLE tbl_name
-> FIELDS TERMINATED BY ',' ENCLOSED BY '"'
-> LINES TERMINATED BY '\n';
Any of the field- or line-handling options can specify an empty string (''). If not empty, the FIELDS [OPTIONALLY] ENCLOSED BY and FIELDS ESCAPED BY values must be a single character. The FIELDS TERMINATED BY, LINES STARTING BY, and LINES TERMINATED BY values can be more than one character. For example, to write lines that are terminated by carriage return/linefeed pairs, or to read a file containing such lines, specify a LINES TERMINATED BY '\r\n' clause.
To read a file containing jokes that are separated by lines consisting of of %%, you can do this
mysql> CREATE TABLE jokes
-> (a INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
-> joke TEXT NOT NULL);
mysql> LOAD DATA INFILE '/tmp/jokes.txt' INTO TABLE jokes
-> FIELDS TERMINATED BY ''
-> LINES TERMINATED BY '\n%%\n' (joke);
FIELDS [OPTIONALLY] ENCLOSED BY controls quoting of fields. For output (SELECT ... INTO OUTFILE), if you omit the word OPTIONALLY, all fields are enclosed by the ENCLOSED BY character. An example of such output (using a comma as the field delimiter) is shown here:
"1","a string","100.20" "2","a string containing a , comma","102.20" "3","a string containing a \" quote","102.20" "4","a string containing a \", quote and comma","102.20"
If you specify OPTIONALLY, the ENCLOSED BY character is used only to enclose CHAR and VARCHAR fields:
1,"a string",100.20 2,"a string containing a , comma",102.20 3,"a string containing a \" quote",102.20 4,"a string containing a \", quote and comma",102.20
Note that occurrences of the ENCLOSED BY character within a field value are escaped by prefixing them with the ESCAPED BY character. Also note that if you specify an empty ESCAPED BY value, it is possible to generate output that cannot be read properly by LOAD DATA INFILE. For example, the preceding output just shown would appear as follows if the escape character is empty. Observe that the second field in the fourth line contains a comma following the quote, which (erroneously) appears to terminate the field:
1,"a string",100.20 2,"a string containing a , comma",102.20 3,"a string containing a " quote",102.20 4,"a string containing a ", quote and comma",102.20
For input, the ENCLOSED BY character, if present, is stripped from the ends of field values. (This is true whether or not OPTIONALLY is specified; OPTIONALLY has no effect on input interpretation.) Occurrences of the ENCLOSED BY character preceded by the ESCAPED BY character are interpreted as part of the current field value.
If the field begins with the ENCLOSED BY character, instances of that character are recognized as terminating a field value only if followed by the field or line TERMINATED BY sequence. To avoid ambiguity, occurrences of the ENCLOSED BY character within a field value can be doubled and will be interpreted as a single instance of the character. For example, if ENCLOSED BY '"' is specified, quotes are handled as shown here:
"The ""BIG"" boss" -> The "BIG" boss The "BIG" boss -> The "BIG" boss The ""BIG"" boss -> The ""BIG"" boss
FIELDS ESCAPED BY controls how to write or read special characters. If the FIELDS ESCAPED BY character is not empty, it is used to prefix the following characters on output:
The FIELDS ESCAPED BY character
The FIELDS [OPTIONALLY] ENCLOSED BY character
The first character of the FIELDS TERMINATED BY and LINES TERMINATED BY values
ASCII 0 (what is actually written following the escape character is ASCII ‘0’, not a zero-valued byte)
If the FIELDS ESCAPED BY character is empty, no characters are escaped and NULL is output as NULL, not \N. It is probably not a good idea to specify an empty escape character, particularly if field values in your data contain any of the characters in the list just given.
For input, if the FIELDS ESCAPED BY character is not empty, occurrences of that character are stripped and the following character is taken literally as part of a field value. The exceptions are an escaped ‘0’ or ‘N’ (for example, \0 or \N if the escape character is ‘\’). These sequences are interpreted as ASCII NUL (a zero-valued byte) and NULL. The rules for NULL handling are described later in this section.
For more information about ‘\’-escape syntax, see the section called “Literal Values”.
In certain cases, field- and line-handling options interact:
If LINES TERMINATED BY is an empty string and FIELDS TERMINATED BY is non-empty, lines are also terminated with FIELDS TERMINATED BY.
If the FIELDS TERMINATED BY and FIELDS ENCLOSED BY values are both empty (''), a fixed-row (non-delimited) format is used. With fixed-row format, no delimiters are used between fields (but you can still have a line terminator). Instead, column values are written and read using the “display” widths of the columns. For example, if a column is declared as INT(7), values for the column are written using seven-character fields. On input, values for the column are obtained by reading seven characters.
LINES TERMINATED BY is still used to separate lines. If a line doesn't contain all fields, the rest of the columns are set to their default values. If you don't have a line terminator, you should set this to ''. In this case, the text file must contain all fields for each row.
Fixed-row format also affects handling of NULL values, as described later. Note that fixed-size format will not work if you are using a multi-byte character set.
Handling of NULL values varies according to the FIELDS and LINES options in use:
For the default FIELDS and LINES values, NULL is written as a field value of \N for output, and a field value of \N is read as NULL for input (assuming that the ESCAPED BY character is ‘\’).
If FIELDS ENCLOSED BY is not empty, a field containing the literal word NULL as its value is read as a NULL value. This differs from the word NULL enclosed within FIELDS ENCLOSED BY characters, which is read as the string 'NULL'.
If FIELDS ESCAPED BY is empty, NULL is written as the word NULL.
With fixed-row format (which happens when FIELDS TERMINATED BY and FIELDS ENCLOSED BY are both empty), NULL is written as an empty string. Note that this causes both NULL values and empty strings in the table to be indistinguishable when written to the file because they are both written as empty strings. If you need to be able to tell the two apart when reading the file back in, you should not use fixed-row format.
Some cases are not supported by LOAD DATA INFILE:
Fixed-size rows (FIELDS TERMINATED BY and FIELDS ENCLOSED BY both empty) and BLOB or TEXT columns.
If you specify one separator that is the same as or a prefix of another, LOAD DATA INFILE won't be able to interpret the input properly. For example, the following FIELDS clause would cause problems:
FIELDS TERMINATED BY '"' ENCLOSED BY '"'
If FIELDS ESCAPED BY is empty, a field value that contains an occurrence of FIELDS ENCLOSED BY or LINES TERMINATED BY followed by the FIELDS TERMINATED BY value will cause LOAD DATA INFILE to stop reading a field or line too early. This happens because LOAD DATA INFILE cannot properly determine where the field or line value ends.
The following example loads all columns of the persondata table:
mysql> LOAD DATA INFILE 'persondata.txt' INTO TABLE persondata;
By default, when no column list is provided at the end of the LOAD DATA INFILE statement, input lines are expected to contain a field for each table column. If you want to load only some of a table's columns, specify a column list:
mysql> LOAD DATA INFILE 'persondata.txt'
-> INTO TABLE persondata (col1,col2,...);
You must also specify a column list if the order of the fields in the input file differs from the order of the columns in the table. Otherwise, MySQL cannot tell how to match up input fields with table columns.
If an input line has too many fields, the extra fields are ignored and the number of warnings is incremented.
If an input line has too few fields, the table columns for which input fields are missing are set to their default values. Default value assignment is described in CREATE TABLE.
An empty field value is interpreted differently than if the field value is missing:
For string types, the column is set to the empty string.
For numeric types, the column is set to 0.
For date and time types, the column is set to the appropriate “zero” value for the type. See the section called “Date and Time Types”.
These are the same values that result if you assign an empty string explicitly to a string, numeric, or date or time type explicitly in an INSERT or UPDATE statement.
TIMESTAMP columns are set to the current date and time only if there is a NULL value for the column (that is, \N), or (for the first TIMESTAMP column only) if the TIMESTAMP column is omitted from the field list when a field list is specified.
LOAD DATA INFILE regards all input as strings, so you can't use numeric values for ENUM or SET columns the way you can with INSERT statements. All ENUM and SET values must be specified as strings!
When the LOAD DATA INFILE statement finishes, it returns an information string in the following format:
Records: 1 Deleted: 0 Skipped: 0 Warnings: 0
If you are using the C API, you can get information about the statement by calling the mysql_info() function. See mysql_info().
Warnings occur under the same circumstances as when values are inserted via the INSERT statement (see INSERT), except that LOAD DATA INFILE also generates warnings when there are too few or too many fields in the input row. The warnings are not stored anywhere; the number of warnings can be used only as an indication of whether everything went well.
From MySQL 4.1.1 on, you can use SHOW WARNINGS to get a list of the first max_error_count warnings as information about what went wrong. See SHOW WARNINGS.
Before MySQL 4.1.1, only a warning count is available to indicate that something went wrong. If you get warnings and want to know exactly why you got them, one way to do this is to dump the table into another file using SELECT ... INTO OUTFILE and compare the file to your original input file.
REPLACE [LOW_PRIORITY | DELAYED]
[INTO] tbl_name [(col_name,...)]
VALUES ({expr | DEFAULT},...),(...),...
Or:
REPLACE [LOW_PRIORITY | DELAYED]
[INTO] tbl_name
SET col_name={expr | DEFAULT}, ...
Or:
REPLACE [LOW_PRIORITY | DELAYED]
[INTO] tbl_name [(col_name,...)]
SELECT ...
REPLACE works exactly like INSERT, except that if an old record in the table has the same value as a new record for a PRIMARY KEY or a UNIQUE index, the old record is deleted before the new record is inserted. See INSERT.
Note that unless the table has a PRIMARY KEY or UNIQUE index, using a REPLACE statement makes no sense. It becomes equivalent to INSERT, because there is no index to be used to determine whether a new row duplicates another.
Values for all columns are taken from the values specified in the REPLACE statement. Any missing columns are set to their default values, just as happens for INSERT. You can't refer to values from the old row and use them in the new row. It appeared that you could do this in some old MySQL versions, but that was a bug that has been corrected.
To be able to use REPLACE, you must have INSERT and DELETE privileges for the table. The REPLACE statement returns a count to indicate the number of rows affected. This is the sum of the rows deleted and inserted. If the count is 1 for a single-row REPLACE, a row was inserted and no rows were deleted. If the count is greater than 1, one or more old rows were deleted before the new row was inserted. It is possible for a single row to replace more than one old row if the table contains multiple unique indexes and the new row duplicates values for different old rows in different unique indexes.
The affected-rows count makes it easy to determine whether REPLACE only added a row or whether it also replaced any rows: Check whether the count is 1 (added) or greater (replaced).
If you are using the C API, the affected-rows count can be obtained using the mysql_affected_rows() function.
Currently, you cannot replace into a table and select from the same table in a subquery.
Here follows in more detail the algorithm that is used (it is also used with LOAD DATA ... REPLACE):
Try to insert the new row into the table
While the insertion fails because a duplicate-key error occurs for a primary or unique key:
Delete from the table the conflicting row that has the duplicate key value
Try again to insert the new row into the table
SELECT
[ALL | DISTINCT | DISTINCTROW ]
[HIGH_PRIORITY]
[STRAIGHT_JOIN]
[SQL_SMALL_RESULT] [SQL_BIG_RESULT] [SQL_BUFFER_RESULT]
[SQL_CACHE | SQL_NO_CACHE] [SQL_CALC_FOUND_ROWS]
select_expr, ...
[INTO OUTFILE 'file_name' export_options
| INTO DUMPFILE 'file_name']
[FROM table_references
[WHERE where_definition]
[GROUP BY {col_name | expr | position}
[ASC | DESC], ... [WITH ROLLUP]]
[HAVING where_definition]
[ORDER BY {col_name | expr | position}
[ASC | DESC] , ...]
[LIMIT {[offset,] row_count | row_count OFFSET offset}]
[PROCEDURE procedure_name(argument_list)]
[FOR UPDATE | LOCK IN SHARE MODE]]
SELECT is used to retrieve rows selected from one or more tables. Support for UNION statements and subqueries is available as of MySQL 4.0 and 4.1, respectively. See UNION and the section called “Subquery Syntax”.
Each select_expr indicates a column you want to retrieve.
table_references indicates the table or tables from which to retrieve rows. Its syntax is described in JOIN.
where_definition consists of the keyword WHERE followed by an expression that indicates the condition or conditions that rows must satisfy to be selected.
SELECT can also be used to retrieve rows computed without reference to any table. For example:
mysql> SELECT 1 + 1;
-> 2
All clauses used must be given in exactly the order shown in the syntax description. For example, a HAVING clause must come after any GROUP BY clause and before any ORDER BY clause.
A select_expr can be given an alias using AS alias_name. The alias is used as the expression's column name and can be used in GROUP BY, ORDER BY, or HAVING clauses. For example:
mysql> SELECT CONCAT(last_name,', ',first_name) AS full_name
-> FROM mytable ORDER BY full_name;
The AS keyword is optional when aliasing a select_expr. The preceding example could have been written like this:
mysql> SELECT CONCAT(last_name,', ',first_name) full_name
-> FROM mytable ORDER BY full_name;
Because the AS is optional, a subtle problem can occur if you forget the comma between two select_expr expressions: MySQL interprets the second as an alias name. For example, in the following statement, columnb is treated as an alias name:
mysql> SELECT columna columnb FROM mytable;
It is not allowable to use a column alias in a WHERE clause, because the column value might not yet be determined when the WHERE clause is executed. See the section called “Problems with Column Aliases”.
The FROM table_references clause indicates the tables from which to retrieve rows. If you name more than one table, you are performing a join. For information on join syntax, see JOIN. For each table specified, you can optionally specify an alias.
tbl_name [[AS] alias]
[[USE INDEX (key_list)]
| [IGNORE INDEX (key_list)]
| [FORCE INDEX (key_list)]]
The use of USE INDEX, IGNORE INDEX, FORCE INDEX to give the optimizer hints about how to choose indexes is described in JOIN.
In MySQL 4.0.14, you can use SET max_seeks_for_key=value as an alternative way to force MySQL to prefer key scans instead of table scans.
You can refer to a table within the current database as tbl_name (within the current database), or as db_name.tbl_name to explicitly specify a database. You can refer to a column as col_name, tbl_name.col_name, or db_name.tbl_name.col_name. You need not specify a tbl_name or db_name.tbl_name prefix for a column reference unless the reference would be ambiguous. See the section called “Database, Table, Index, Column, and Alias Names” for examples of ambiguity that require the more explicit column reference forms.
From MySQL 4.1.0 on, you are allowed to specify DUAL as a dummy table name in situations where no tables are referenced:
mysql> SELECT 1 + 1 FROM DUAL;
-> 2
DUAL is purely a compatibility feature. Some other servers require this syntax.
A table reference can be aliased using tbl_name AS alias_name or tbl_name alias_name:
mysql> SELECT t1.name, t2.salary FROM employee AS t1, info AS t2
-> WHERE t1.name = t2.name;
mysql> SELECT t1.name, t2.salary FROM employee t1, info t2
-> WHERE t1.name = t2.name;
In the WHERE clause, you can use any of the functions that MySQL supports, except for aggregate (summary) functions. See Chapter 13, Functions and Operators.
Columns selected for output can be referred to in ORDER BY and GROUP BY clauses using column names, column aliases, or column positions. Column positions are integers and begin with 1:
mysql> SELECT college, region, seed FROM tournament
-> ORDER BY region, seed;
mysql> SELECT college, region AS r, seed AS s FROM tournament
-> ORDER BY r, s;
mysql> SELECT college, region, seed FROM tournament
-> ORDER BY 2, 3;
To sort in reverse order, add the DESC (descending) keyword to the name of the column in the ORDER BY clause that you are sorting by. The default is ascending order; this can be specified explicitly using the ASC keyword.
Use of column positions is deprecated because the syntax has been removed from the SQL standard.
If you use GROUP BY, output rows are sorted according to the GROUP BY columns as if you had an ORDER BY for the same columns. MySQL has extended the GROUP BY clause as of version 3.23.34 so that you can also specify ASC and DESC after columns named in the clause:
SELECT a, COUNT(b) FROM test_table GROUP BY a DESC
MySQL extends the use of GROUP BY to allow you to select fields that are not mentioned in the GROUP BY clause. If you are not getting the results you expect from your query, please read the GROUP BY description. See the section called “Functions and Modifiers for Use with GROUP BY Clauses”.
As of MySQL 4.1.1, GROUP BY allows a WITH ROLLUP modifier. See GROUP BY Modifiers.
The HAVING clause can refer to any column or alias named in a select_expr. It is applied nearly last, just before items are sent to the client, with no optimization. (LIMIT is applied after HAVING.)
Don't use HAVING for items that should be in the WHERE clause. For example, do not write this:
mysql> SELECT col_name FROM tbl_name HAVING col_name > 0;
Write this instead:
mysql> SELECT col_name FROM tbl_name WHERE col_name > 0;
The HAVING clause can refer to aggregate functions, which the WHERE clause cannot:
mysql> SELECT user, MAX(salary) FROM users
-> GROUP BY user HAVING MAX(salary)>10;
However, that does not work in older MySQL servers (before version 3.22.5). Instead, you can use a column alias in the select list and refer to the alias in the HAVING clause:
mysql> SELECT user, MAX(salary) AS max_salary FROM users
-> GROUP BY user HAVING max_salary>10;
The LIMIT clause can be used to constrain the number of rows returned by the SELECT statement. LIMIT takes one or two numeric arguments, which must be integer constants.
With two arguments, the first argument specifies the offset of the first row to return, and the second specifies the maximum number of rows to return. The offset of the initial row is 0 (not 1):
mysql> SELECT * FROM table LIMIT 5,10; # Retrieve rows 6-15
For compatibility with PostgreSQL, MySQL also supports the LIMIT row_count OFFSET offset syntax.
To retrieve all rows from a certain offset up to the end of the result set, you can use some large number for the second parameter. This statement retrieves all rows from the 96th row to the last:
mysql> SELECT * FROM table LIMIT 95,18446744073709551615;
With one argument, the value specifies the number of rows to return from the beginning of the result set:
mysql> SELECT * FROM table LIMIT 5; # Retrieve first 5 rows
In other words, LIMIT n is equivalent to LIMIT 0,n.
The SELECT ... INTO OUTFILE 'file_name' form of SELECT writes the selected rows to a file. The file is created on the server host, so you must have the FILE privilege to use this syntax. The file cannot already exist, which among other things prevents files such as /etc/passwd and database tables from being destroyed.
The SELECT ... INTO OUTFILE statement is intended primarily to let you very quickly dump a table on the server machine. If you want to create the resulting file on some client host other than the server host, you can't use SELECT ... INTO OUTFILE. In that case, you should instead use some command like mysql -e "SELECT ..." > file_name on the client host to generate the file.
SELECT ... INTO OUTFILE is the complement of LOAD DATA INFILE; the syntax for the export_options part of the statement consists of the same FIELDS and LINES clauses that are used with the LOAD DATA INFILE statement. See LOAD DATA.
FIELDS ESCAPED BY controls how to write special characters. If the FIELDS ESCAPED BY character is not empty, it is used to prefix the following characters on output:
The FIELDS ESCAPED BY character
The FIELDS [OPTIONALLY] ENCLOSED BY character
The first character of the FIELDS TERMINATED BY and LINES TERMINATED BY values
ASCII 0 (what is actually written following the escape character is ASCII ‘0’, not a zero-valued byte)
If the FIELDS ESCAPED BY character is empty, no characters are escaped and NULL is output as NULL, not \N. It is probably not a good idea to specify an empty escape character, particularly if field values in your data contain any of the characters in the list just given.
The reason for the above is that you must escape any FIELDS TERMINATED BY, ENCLOSED BY, ESCAPED BY, or LINES TERMINATED BY characters to reliably be able to read the file back. ASCII NUL is escaped to make it easier to view with some pagers.
The resulting file doesn't have to conform to SQL syntax, so nothing else need be escaped.
Here is an example that produces a file in the comma-separated values format used by many programs:
SELECT a,b,a+b INTO OUTFILE '/tmp/result.text' FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"' LINES TERMINATED BY '\n' FROM test_table;
If you use INTO DUMPFILE instead of INTO OUTFILE, MySQL writes only one row into the file, without any column or line termination and without performing any escape processing. This is useful if you want to store a BLOB value in a file.
Note: Any file created by INTO OUTFILE or INTO DUMPFILE is writable by all users on the server host. The reason for this is that the MySQL server can't create a file that is owned by anyone other than the user it's running as (you should never run mysqld as root). The file thus must be world-writable so that you can manipulate its contents.
A PROCEDURE clause names a procedure that should process the data in the result set. For an example, see the section called “Procedure Analyse”.
If you use FOR UPDATE on a storage engine that uses page or row locks, rows examined by the query are write-locked until the end of the current transaction. Using IN SHARE MODE sets a shared lock that prevents other transactions from updating or deleting the examined rows. See InnoDB locking reads.
Following the SELECT keyword, you can give a number of options that affect the operation of the statement.
The ALL, DISTINCT, and DISTINCTROW options specify whether duplicate rows should be returned. If none of these options are given, the default is ALL (all matching rows are returned). DISTINCT and DISTINCTROW are synonyms and specify that duplicate rows in the result set should be removed.
HIGH_PRIORITY, STRAIGHT_JOIN, and options beginning with SQL_ are MySQL extensions to standard SQL.
HIGH_PRIORITY will give the SELECT higher priority than a statement that updates a table. You should use this only for queries that are very fast and must be done at once. A SELECT HIGH_PRIORITY query that is issued while the table is locked for reading will run even if there is already an update statement waiting for the table to be free.
HIGH_PRIORITY cannot be used with SELECT statements that are part of a UNION.
STRAIGHT_JOIN forces the optimizer to join the tables in the order in which they are listed in the FROM clause. You can use this to speed up a query if the optimizer joins the tables in non-optimal order. See EXPLAIN. STRAIGHT_JOIN also can be used in the table_references list. See JOIN.
SQL_BIG_RESULT can be used with GROUP BY or DISTINCT to tell the optimizer that the result set will have many rows. In this case, MySQL will directly use disk-based temporary tables if needed. MySQL will also, in this case, prefer sorting to using a temporary table with a key on the GROUP BY elements.
SQL_BUFFER_RESULT forces the result to be put into a temporary table. This helps MySQL free the table locks early and helps in cases where it takes a long time to send the result set to the client.
SQL_SMALL_RESULT can be used with GROUP BY or DISTINCT to tell the optimizer that the result set will be small. In this case, MySQL uses fast temporary tables to store the resulting table instead of using sorting. In MySQL 3.23 and up, this shouldn't normally be needed.
SQL_CALC_FOUND_ROWS (available in MySQL 4.0.0 and up) tells MySQL to calculate how many rows there would be in the result set, disregarding any LIMIT clause. The number of rows can then be retrieved with SELECT FOUND_ROWS(). See the section called “Information Functions”.
Before MySQL 4.1.0, this option does not work with LIMIT 0, which is optimized to return instantly (resulting in a row count of 0). See LIMIT optimization.
SQL_CACHE tells MySQL to store the query result in the query cache if you are using a query_cache_type value of 2 or DEMAND. For a query that uses UNION or subqueries, this option takes effect to be used in any SELECT of the query. See the section called “The MySQL Query Cache”.
SQL_NO_CACHE tells MySQL not to store the query result in the query cache. See the section called “The MySQL Query Cache”. For a query that uses UNION or subqueries, this option takes effect to be used in any SELECT of the query.
MySQL supports the following JOIN syntaxes for the table_references part of SELECT statements and multiple-table DELETE and UPDATE statements:
table_reference, table_reference
table_reference [INNER | CROSS] JOIN table_reference [join_condition]
table_reference STRAIGHT_JOIN table_reference
table_reference LEFT [OUTER] JOIN table_reference [join_condition]
table_reference NATURAL [LEFT [OUTER]] JOIN table_reference
{ OJ table_reference LEFT OUTER JOIN table_reference
ON conditional_expr }
table_reference RIGHT [OUTER] JOIN table_reference [join_condition]
table_reference NATURAL [RIGHT [OUTER]] JOIN table_reference
table_reference is defined as:
tbl_name [[AS] alias]
[[USE INDEX (key_list)]
| [IGNORE INDEX (key_list)]
| [FORCE INDEX (key_list)]]
join_condition is defined as:
ON conditional_expr | USING (column_list)
You should generally not have any conditions in the ON part that are used to restrict which rows you want in the result set, but rather specify these conditions in the WHERE clause. There are exceptions to this rule.
Note that INNER JOIN syntax allows a join_condition only from MySQL 3.23.17 on. The same is true for JOIN and CROSS JOIN only as of MySQL 4.0.11.
The { OJ ... LEFT OUTER JOIN ...} syntax shown in the preceding list exists only for compatibility with ODBC.
A table reference can be aliased using tbl_name AS alias_name or tbl_name alias_name:
mysql> SELECT t1.name, t2.salary FROM employee AS t1, info AS t2
-> WHERE t1.name = t2.name;
mysql> SELECT t1.name, t2.salary FROM employee t1, info t2
-> WHERE t1.name = t2.name;
The ON conditional is any conditional expression of the form that can be used in a WHERE clause.
If there is no matching record for the right table in the ON or USING part in a LEFT JOIN, a row with all columns set to NULL is used for the right table. You can use this fact to find records in a table that have no counterpart in another table:
mysql> SELECT table1.* FROM table1
-> LEFT JOIN table2 ON table1.id=table2.id
-> WHERE table2.id IS NULL;
This example finds all rows in table1 with an id value that is not present in table2 (that is, all rows in table1 with no corresponding row in table2). This assumes that table2.id is declared NOT NULL. See LEFT JOIN optimization.
The USING (column_list) clause names a list of columns that must exist in both tables. The following two clauses are semantically identical:
a LEFT JOIN b USING (c1,c2,c3) a LEFT JOIN b ON a.c1=b.c1 AND a.c2=b.c2 AND a.c3=b.c3
The NATURAL [LEFT] JOIN of two tables is defined to be semantically equivalent to an INNER JOIN or a LEFT JOIN with a USING clause that names all columns that exist in both tables.
INNER JOIN and , (comma) are semantically equivalent in the absence of a join condition: both will produce a Cartesian product between the specified tables (that is, each and every row in the first table will be joined onto all rows in the second table).
RIGHT JOIN works analogously to LEFT JOIN. To keep code portable across databases, it's recommended to use LEFT JOIN instead of RIGHT JOIN.
STRAIGHT_JOIN is identical to JOIN, except that the left table is always read before the right table. This can be used for those (few) cases for which the join optimizer puts the tables in the wrong order.
As of MySQL 3.23.12, you can give hints about which index MySQL should use when retrieving information from a table. By specifying USE INDEX (key_list), you can tell MySQL to use only one of the possible indexes to find rows in the table. The alternative syntax IGNORE INDEX (key_list) can be used to tell MySQL to not use some particular index. These hints are useful if EXPLAIN shows that MySQL is using the wrong index from the list of possible indexes.
From MySQL 4.0.9 on, you can also use FORCE INDEX. This acts likes USE INDEX (key_list) but with the addition that a table scan is assumed to be very expensive. In other words, a table scan will only be used if there is no way to use one of the given indexes to find rows in the table.
USE KEY, IGNORE KEY, and FORCE KEY are synonyms for USE INDEX, IGNORE INDEX, and FORCE INDEX.
Note: USE INDEX, IGNORE INDEX, and FORCE INDEX only affect which indexes are used when MySQL decides how to find rows in the table and how to do the join. They do not affect whether an index will be used when resolving an ORDER BY or GROUP BY.
Some join examples:
mysql> SELECT * FROM table1,table2 WHERE table1.id=table2.id;
mysql> SELECT * FROM table1 LEFT JOIN table2 ON table1.id=table2.id;
mysql> SELECT * FROM table1 LEFT JOIN table2 USING (id);
mysql> SELECT * FROM table1 LEFT JOIN table2 ON table1.id=table2.id
-> LEFT JOIN table3 ON table2.id=table3.id;
mysql> SELECT * FROM table1 USE INDEX (key1,key2)
-> WHERE key1=1 AND key2=2 AND key3=3;
mysql> SELECT * FROM table1 IGNORE INDEX (key3)
-> WHERE key1=1 AND key2=2 AND key3=3;
SELECT ... UNION [ALL | DISTINCT] SELECT ... [UNION [ALL | DISTINCT] SELECT ...]
UNION is used to combine the result from many SELECT statements into one result set. UNION is available from MySQL 4.0.0 on.
Selected columns listed in corresponding positions of each SELECT statement should have the same type. (For example, the first column selected by the first statement should have the same type as the first column selected by the other statements.) The column names used in the first SELECT statement are used as the column names for the results returned. The SELECT statements are normal select statements, but with the following restrictions:
Only the last SELECT statement can have INTO OUTFILE.
HIGH_PRIORITY cannot be used with SELECT statements that are part of a UNION. If you specify it for the first SELECT, it has no effect. If you specify it for any subsequent SELECT statements, a syntax error results.
If you don't use the keyword ALL for the UNION, all returned rows will be unique, as if you had done a DISTINCT for the total result set. If you specify ALL, you will get all matching rows from all the used SELECT statements.
The DISTINCT keyword is an optional word (introduced in MySQL 4.0.17). It does nothing, but is allowed in the syntax as required by the SQL standard.
Before MySQL 4.1.2, you cannot mix UNION ALL and UNION DISTINCT in the same query. If you use ALL for one UNION, it is used for all of them. As of MySQL 4.1.2, mixed UNION types are treated such that a DISTINCT union overrides any ALL union to its left. A DISTINCT union can be produced explicitly by using UNION DISTINCT or implicitly by using UNION with no following DISTINCT or ALL keyword.
If you want to use an ORDER BY to sort the entire UNION result, you should use parentheses:
(SELECT a FROM tbl_name WHERE a=10 AND B=1 ORDER BY a LIMIT 10) UNION (SELECT a FROM tbl_name WHERE a=11 AND B=2 ORDER BY a LIMIT 10) ORDER BY a;
This kind of ORDER BY cannot use column references that include a table name (that is, names in tbl_name.col_name format). Instead, provide a column alias in the first SELECT statement and refer to the alias in the ORDER BY, or else refer to the column in the ORDER BY using its column position. (An alias is preferable because use of column positions is deprecated.)
The types and lengths of the columns in the result set of a UNION take into account the values retrieved by all the SELECT statements. Before MySQL 4.1.1, a limitation of UNION is that only the values from the first SELECT are used to determine result column types and lengths. This could result in value truncation if, for example, the first SELECT retrieves shorter values than the second SELECT:
mysql> SELECT REPEAT('a',1) UNION SELECT REPEAT('b',10);
+---------------+
| REPEAT('a',1) |
+---------------+
| a |
| b |
+---------------+
That limitation has been removed as of MySQL 4.1.1:
mysql> SELECT REPEAT('a',1) UNION SELECT REPEAT('b',10);
+---------------+
| REPEAT('a',1) |
+---------------+
| a |
| bbbbbbbbbb |
+---------------+
A subquery is a SELECT statement inside another statement.
Starting with MySQL 4.1, all subquery forms and operations that the SQL standard requires are supported, as well as a few features that are MySQL-specific.
With earlier MySQL versions, it was necessary to work around or avoid the use of subqueries, but people starting to write code now will find that subqueries are a very useful part of the MySQL toolkit.
For MySQL versions prior to 4.1, most subqueries can be successfully rewritten using joins and other methods. See the section called “Rewriting Subqueries as Joins for Earlier MySQL Versions”.
Here is an example of a subquery:
SELECT * FROM t1 WHERE column1 = (SELECT column1 FROM t2);
In this example, SELECT * FROM t1 ... is the outer query (or outer statement), and (SELECT column1 FROM t2) is the subquery. We say that the subquery is nested in the outer query, and in fact it's possible to nest subqueries within other subqueries, to a great depth. A subquery must always appear within parentheses.
The main advantages of subqueries are:
They allow queries that are structured so that it's possible to isolate each part of a statement.
They provide alternative ways to perform operations that would otherwise require complex joins and unions.
They are, in many people's opinion, readable. Indeed, it was the innovation of subqueries that gave people the original idea of calling the early SQL “Structured Query Language.”
Here is an example statement that shows the major points about subquery syntax as specified by the SQL standard and supported in MySQL:
DELETE FROM t1
WHERE s11 > ANY
(SELECT COUNT(*) /* no hint */ FROM t2
WHERE NOT EXISTS
(SELECT * FROM t3
WHERE ROW(5*t2.s1,77)=
(SELECT 50,11*s1 FROM t4 UNION SELECT 50,77 FROM
(SELECT * FROM t5) AS t5)));
Currently, you cannot modify a table and select from the same table in a subquery. This applies to statements such as DELETE, INSERT, REPLACE, and UPDATE.
In its simplest form (the scalar subquery as opposed to the row or table subqueries that are discussed later), a subquery is a simple operand. Thus, you can use it wherever a column value or literal is legal, and you can expect it to have those characteristics that all operands have: a data type, a length, an indication whether it can be NULL, and so on. For example:
CREATE TABLE t1 (s1 INT, s2 CHAR(5) NOT NULL); SELECT (SELECT s2 FROM t1);
The subquery in this SELECT has a data type of CHAR, a length of 5, a character set and collation equal to the defaults in effect at CREATE TABLE time, and an indication that the value in the column can be NULL. In fact, almost all subqueries can be NULL, because if the table is empty, as in the example, the value of the subquery will be NULL. There are few restrictions.
A subquery's outer statement can be any one of: SELECT, INSERT, UPDATE, DELETE, SET, or DO.
A subquery can contain any of the keywords or clauses that an ordinary SELECT can contain: DISTINCT, GROUP BY, ORDER BY, LIMIT, joins, hints, UNION constructs, comments, functions, and so on.
So, when you see examples in the following sections that contain the rather spartan construct (SELECT column1 FROM t1), imagine that your own code will contain much more diverse and complex constructions.
For example, suppose that we make two tables:
CREATE TABLE t1 (s1 INT); INSERT INTO t1 VALUES (1); CREATE TABLE t2 (s1 INT); INSERT INTO t2 VALUES (2);
Then perform a SELECT:
SELECT (SELECT s1 FROM t2) FROM t1;
The result will be 2 because there is a row in t2 containing a column s1 that has a value of 2.
The subquery can be part of an expression. If it is an operand for a function, don't forget the parentheses. For example:
SELECT UPPER((SELECT s1 FROM t1)) FROM t2;
The most common use of a subquery is in the form:
non_subquery_operand comparison_operator (subquery)
Where comparison_operator is one of:
= > < >= <= <>
For example:
... 'a' = (SELECT column1 FROM t1)
At one time the only legal place for a subquery was on the right side of a comparison, and you might still find some old DBMSs that insist on this.
Here is an example of a common-form subquery comparison that you cannot do with a join. It finds all the values in table t1 that are equal to a maximum value in table t2:
SELECT column1 FROM t1
WHERE column1 = (SELECT MAX(column2) FROM t2);
Here is another example, which again is impossible with a join because it involves aggregating for one of the tables. It finds all rows in table t1 containing a value that occurs twice in a given column:
SELECT * FROM t1 AS t
WHERE 2 = (SELECT COUNT(*) FROM t1 WHERE t1.id = t.id);
Syntax:
operand comparison_operator ANY (subquery) operand IN (subquery) operand comparison_operator SOME (subquery)
The ANY keyword, which must follow a comparison operator, means “return TRUE if the comparison is TRUE for ANY of the rows that the subquery returns.” For example:
SELECT s1 FROM t1 WHERE s1 > ANY (SELECT s1 FROM t2);
Suppose that there is a row in table t1 containing (10). The expression is TRUE if table t2 contains (21,14,7) because there is a value 7 in t2 that is less than 10. The expression is FALSE if table t2 contains (20,10), or if table t2 is empty. The expression is UNKNOWN if table t2 contains (NULL,NULL,NULL).
The word IN is an alias for = ANY. Thus these two statements are the same:
SELECT s1 FROM t1 WHERE s1 = ANY (SELECT s1 FROM t2); SELECT s1 FROM t1 WHERE s1 IN (SELECT s1 FROM t2);
However, NOT IN is not an alias for <> ANY, but for <> ALL. See ALL subqueries.
The word SOME is an alias for ANY. Thus these two statements are the same:
SELECT s1 FROM t1 WHERE s1 <> ANY (SELECT s1 FROM t2); SELECT s1 FROM t1 WHERE s1 <> SOME (SELECT s1 FROM t2);
Use of the word SOME is rare, but this example shows why it might be useful. To most people's ears, the English phrase “a is not equal to any b” means “there is no b which is equal to a,” but that isn't what is meant by the SQL syntax. Using <> SOME instead helps ensure that everyone understands the true meaning of the query.
Syntax:
operand comparison_operator ALL (subquery)
The word ALL, which must follow a comparison operator, means “return TRUE if the comparison is TRUE for ALL of the rows that the subquery returns.” For example:
SELECT s1 FROM t1 WHERE s1 > ALL (SELECT s1 FROM t2);
Suppose that there is a row in table t1 containing (10). The expression is TRUE if table t2 contains (-5,0,+5) because 10 is greater than all three values in t2. The expression is FALSE if table t2 contains (12,6,NULL,-100) because there is a single value 12 in table t2 that is greater than 10. The expression is UNKNOWN if table t2 contains (0,NULL,1).
Finally, if table t2 is empty, the result is TRUE. You might think the result should be UNKNOWN, but sorry, it's TRUE. So, rather oddly, the following statement is TRUE when table t2 is empty:
SELECT * FROM t1 WHERE 1 > ALL (SELECT s1 FROM t2);
But this statement is UNKNOWN when table t2 is empty:
SELECT * FROM t1 WHERE 1 > (SELECT s1 FROM t2);
In addition, the following statement is UNKNOWN when table t2 is empty:
SELECT * FROM t1 WHERE 1 > ALL (SELECT MAX(s1) FROM t2);
In general, tables with NULL values and empty tables are edge cases. When writing subquery code, always consider whether you have taken those two possibilities into account.
NOT IN is an alias for <> ALL. Thus these two statements are the same:
SELECT s1 FROM t1 WHERE s1 <> ALL (SELECT s1 FROM t2); SELECT s1 FROM t1 WHERE s1 NOT IN (SELECT s1 FROM t2);
A correlated subquery is a subquery that contains a reference to a table that also appears in the outer query. For example:
SELECT * FROM t1 WHERE column1 = ANY
(SELECT column1 FROM t2 WHERE t2.column2 = t1.column2);
Notice that the subquery contains a reference to a column of t1, even though the subquery's FROM clause doesn't mention a table t1. So, MySQL looks outside the subquery, and finds t1 in the outer query.
Suppose that table t1 contains a row where column1 = 5 and column2 = 6; meanwhile, table t2 contains a row where column1 = 5 and column2 = 7. The simple expression ... WHERE column1 = ANY (SELECT column1 FROM t2) would be TRUE, but in this example, the WHERE clause within the subquery is FALSE (because (5,6) is not equal to (5,7)), so the subquery as a whole is FALSE.
Scoping rule: MySQL evaluates from inside to outside. For example:
SELECT column1 FROM t1 AS x
WHERE x.column1 = (SELECT column1 FROM t2 AS x
WHERE x.column1 = (SELECT column1 FROM t3
WHERE x.column2 = t3.column1));
In this statement, x.column2 must be a column in table t2 because SELECT column1 FROM t2 AS x ... renames t2. It is not a column in table t1 because SELECT column1 FROM t1 ... is an outer query that is farther out.
For subqueries in HAVING or ORDER BY clauses, MySQL also looks for column names in the outer select list.
For certain cases, a correlated subquery is optimized. For example:
val IN (SELECT key_val FROM tbl_name WHERE correlated_condition)
Otherwise, they are inefficient and likely to be slow. Rewriting the query as a join might improve performance.
If a subquery returns any values at all, then EXISTS subquery is TRUE, and NOT EXISTS subquery is FALSE. For example:
SELECT column1 FROM t1 WHERE EXISTS (SELECT * FROM t2);
Traditionally, an EXISTS subquery starts with SELECT *, but it could begin with SELECT 5 or SELECT column1 or anything at all. MySQL ignores the SELECT list in such a subquery, so it doesn't matter.
For the preceding example, if t2 contains any rows, even rows with nothing but NULL values, then the EXISTS condition is TRUE. This is actually an unlikely example, since almost always a [NOT] EXISTS subquery will contain correlations. Here are some more realistic examples:
What kind of store is present in one or more cities?
SELECT DISTINCT store_type FROM Stores
WHERE EXISTS (SELECT * FROM Cities_Stores
WHERE Cities_Stores.store_type = Stores.store_type);
What kind of store is present in no cities?
SELECT DISTINCT store_type FROM Stores
WHERE NOT EXISTS (SELECT * FROM Cities_Stores
WHERE Cities_Stores.store_type = Stores.store_type);
What kind of store is present in all cities?
SELECT DISTINCT store_type FROM Stores S1
WHERE NOT EXISTS (
SELECT * FROM Cities WHERE NOT EXISTS (
SELECT * FROM Cities_Stores
WHERE Cities_Stores.city = Cities.city
AND Cities_Stores.store_type = Stores.store_type));
The last example is a double-nested NOT EXISTS query. That is, it has a NOT EXISTS clause within a NOT EXISTS clause. Formally, it answers the question “does a city exist with a store that is not in Stores?” But it's easier to say that a nested NOT EXISTS answers the question “is x TRUE for all y?”
The discussion to this point has been of column (or scalar) subqueries: subqueries that return a single column value. A row subquery is a subquery variant that returns a single row value and can thus return more than one column value. Here are two examples:
SELECT * FROM t1 WHERE (1,2) = (SELECT column1, column2 FROM t2); SELECT * FROM t1 WHERE ROW(1,2) = (SELECT column1, column2 FROM t2);
The queries here are both TRUE if table t2 has a row where column1 = 1 and column2 = 2.
The expressions (1,2) and ROW(1,2) are sometimes called row constructors. The two are equivalent. They are legal in other contexts, too. For example, the following two statements are semantically equivalent (although currently only the second one can be optimized):
SELECT * FROM t1 WHERE (column1,column2) = (1,1); SELECT * FROM t1 WHERE column1 = 1 AND column2 = 1;
The normal use of row constructors, though, is for comparisons with subqueries that return two or more columns. For example, the following query answers the request, “find all rows in table t1 that also exist in table t2”:
SELECT column1,column2,column3
FROM t1
WHERE (column1,column2,column3) IN
(SELECT column1,column2,column3 FROM t2);
Subqueries are legal in a SELECT statement's FROM clause. The syntax that you'll actually see is:
SELECT ... FROM (subquery) AS name ...
The AS name clause is mandatory, because every table in a FROM clause must have a name. Any columns in the subquery select list must have unique names. You can find this syntax described elsewhere in this manual, where the term used is “derived tables.”
For illustration, assume that you have this table:
CREATE TABLE t1 (s1 INT, s2 CHAR(5), s3 FLOAT);
Here's how to use a subquery in the FROM clause, using the example table:
INSERT INTO t1 VALUES (1,'1',1.0);
INSERT INTO t1 VALUES (2,'2',2.0);
SELECT sb1,sb2,sb3
FROM (SELECT s1 AS sb1, s2 AS sb2, s3*2 AS sb3 FROM t1) AS sb
WHERE sb1 > 1;
Result: 2, '2', 4.0.
Here's another example: Suppose that you want to know the average of a set of sums for a grouped table. This won't work:
SELECT AVG(SUM(column1)) FROM t1 GROUP BY column1;
But this query will provide the desired information:
SELECT AVG(sum_column1)
FROM (SELECT SUM(column1) AS sum_column1
FROM t1 GROUP BY column1) AS t1;
Notice that the column name used within the subquery (sum_column1) is recognized in the outer query.
At the moment, subqueries in the FROM clause cannot be correlated subqueries.
Subquery in the FROM clause will be executed (that is, derived temporary tables will be built) even for the EXPLAIN statement, because upper level queries need information about all tables during optimization phase.
There are some new error returns that apply only to subqueries. This section groups them together because reviewing them will help remind you of some points.
Unsupported subquery syntax:
ERROR 1235 (ER_NOT_SUPPORTED_YET) SQLSTATE = 42000 Message = "This version of MySQL doesn't yet support 'LIMIT & IN/ALL/ANY/SOME subquery'"
This means that statements of the following form will not work, although this happens only in some early versions, such as MySQL 4.1.1:
SELECT * FROM t1 WHERE s1 IN (SELECT s2 FROM t2 ORDER BY s1 LIMIT 1)
Incorrect number of columns from subquery:
ERROR 1241 (ER_OPERAND_COL) SQLSTATE = 21000 Message = "Operand should contain 1 column(s)"
This error will occur in cases like this:
SELECT (SELECT column1, column2 FROM t2) FROM t1;
It's okay to use a subquery that returns multiple columns, if the purpose is comparison. See the section called “Row Subqueries”. But in other contexts, the subquery must be a scalar operand.
Incorrect number of rows from subquery:
ERROR 1242 (ER_SUBSELECT_NO_1_ROW) SQLSTATE = 21000 Message = "Subquery returns more than 1 row"
This error will occur for statements such as the following one, but only when there is more than one row in t2:
SELECT * FROM t1 WHERE column1 = (SELECT column1 FROM t2);
That means this error might occur in code that had been working for years, because somebody happened to make a change that affected the number of rows that the subquery can return. Remember that if the object is to find any number of rows, not just one, then the correct statement would look like this:
SELECT * FROM t1 WHERE column1 = ANY (SELECT column1 FROM t2);
Incorrectly used table in subquery:
Error 1093 (ER_UPDATE_TABLE_USED) SQLSTATE = HY000 Message = "You can't specify target table 'x' for update in FROM clause"
This error will occur in cases like this:
UPDATE t1 SET column2 = (SELECT MAX(column1) FROM t1);
It's okay to use a subquery for assignment within an UPDATE statement, since subqueries are legal in UPDATE and DELETE statements as well as in SELECT statements. However, you cannot use the same table, in this case table t1, for both the subquery's FROM clause and the update target.
Usually, failure of a subquery causes the entire statement to fail.
Development is ongoing, so no optimization tip is reliable for the long term. Some interesting tricks that you might want to play with are:
Use subquery clauses that affect the number or order of the rows in the subquery. For example:
SELECT * FROM t1 WHERE t1.column1 IN (SELECT column1 FROM t2 ORDER BY column1); SELECT * FROM t1 WHERE t1.column1 IN (SELECT DISTINCT column1 FROM t2); SELECT * FROM t1 WHERE EXISTS (SELECT * FROM t2 LIMIT 1);
Replace a join with a subquery. For example, use this query:
SELECT DISTINCT column1 FROM t1 WHERE t1.column1 IN ( SELECT column1 FROM t2);
Instead of this query:
SELECT DISTINCT t1.column1 FROM t1, t2 WHERE t1.column1 = t2.column1;
Move clauses from outside to inside the subquery. For example, use this query:
SELECT * FROM t1 WHERE s1 IN (SELECT s1 FROM t1 UNION ALL SELECT s1 FROM t2);
Instead of this query:
SELECT * FROM t1 WHERE s1 IN (SELECT s1 FROM t1) OR s1 IN (SELECT s1 FROM t2);
For another example, use this query:
SELECT (SELECT column1 + 5 FROM t1) FROM t2;
Instead of this query:
SELECT (SELECT column1 FROM t1) + 5 FROM t2;
Use a row subquery instead of a correlated subquery. For example, use this query:
SELECT * FROM t1 WHERE (column1,column2) IN (SELECT column1,column2 FROM t2);
Instead of this query:
SELECT * FROM t1 WHERE EXISTS (SELECT * FROM t2 WHERE t2.column1=t1.column1 AND t2.column2=t1.column2);
Use NOT (a = ANY (...)) rather than a <> ALL (...).
Use x = ANY (table containing (1,2)) rather than x=1 OR x=2.
Use = ANY rather than EXISTS.
These tricks might cause programs to go faster or slower. Using MySQL facilities like the BENCHMARK() function, you can get an idea about what helps in your own situation. Don't worry too much about transforming to joins except for compatibility with older versions of MySQL before 4.1 that do not support subqueries.
Some optimizations that MySQL itself makes are:
MySQL executes non-correlated subqueries only once. Use EXPLAIN to make sure that a given subquery really is non-correlated.
MySQL rewrites IN/ALL/ANY/SOME subqueries in an attempt to take advantage of the possibility that the select-list columns in the subquery are indexed.
MySQL replaces subqueries of the following form with an index-lookup function, which EXPLAIN will describe as a special join type:
... IN (SELECT indexed_column FROM single_table ...)
MySQL enhances expressions of the following form with an expression involving MIN() or MAX(), unless NULL values or empty sets are involved:
value {ALL|ANY|SOME} {> | < | >= | <=} (non-correlated subquery)
For example, this WHERE clause:
WHERE 5 > ALL (SELECT x FROM t)
might be treated by the optimizer like this:
WHERE 5 > (SELECT MAX(x) FROM t)
There is a chapter titled “How MySQL Transforms Subqueries” in the MySQL Internals Manual. You can obtain this document by downloading the MySQL source package and looking for a file named internals.texi in the Docs directory.
Before MySQL 4.1, only nested queries of the form INSERT ... SELECT ... and REPLACE ... SELECT ... are supported. The IN() construct can be used in other contexts to test membership in a set of values.
It is often possible to rewrite a query without a subquery:
SELECT * FROM t1 WHERE id IN (SELECT id FROM t2);
This can be rewritten as:
SELECT DISTINCT t1.* FROM t1, t2 WHERE t1.id=t2.id;
The queries:
SELECT * FROM t1 WHERE id NOT IN (SELECT id FROM t2); SELECT * FROM t1 WHERE NOT EXISTS (SELECT id FROM t2 WHERE t1.id=t2.id);
Can be rewritten as:
SELECT table1.* FROM table1 LEFT JOIN table2 ON table1.id=table2.id
WHERE table2.id IS NULL;
A LEFT [OUTER] JOIN can be faster than an equivalent subquery because the server might be able to optimize it better—a fact that is not specific to MySQL Server alone. Prior to SQL-92, outer joins did not exist, so subqueries were the only way to do certain things in those bygone days. Today, MySQL Server and many other modern database systems offer a whole range of outer join types.
For more complicated subqueries, you can often create temporary tables to hold the subquery. In some cases, however, this option will not work. The most frequently encountered of these cases arises with DELETE statements, for which standard SQL does not support joins (except in subqueries). For this situation, there are three options available:
The first option is to upgrade to MySQL 4.1, which does support subqueries in DELETE statements.
The second option is to use a procedural programming language (such as Perl or PHP) to submit a SELECT query to obtain the primary keys for the records to be deleted, and then use these values to construct the DELETE statement (DELETE FROM ... WHERE key_col IN (key1, key2, ...)).
The third option is to use interactive SQL to construct a set of DELETE statements automatically, using the MySQL extension CONCAT() (in lieu of the standard || operator). For example:
SELECT
CONCAT('DELETE FROM tab1 WHERE pkid = ', "'", tab1.pkid, "'", ';')
FROM tab1, tab2
WHERE tab1.col1 = tab2.col2;
You can place this query in a script file, use the file as input to one instance of the mysql program, and use the program output as input to a second instance of mysql:
shell> mysql --skip-column-names mydb < myscript.sql | mysql mydb
MySQL Server 4.0 supports multiple-table DELETE statements that can be used to efficiently delete rows based on information from one table or even from many tables at the same time. Multiple-table UPDATE statements are also supported as of MySQL 4.0.
TRUNCATE TABLE tbl_name
TRUNCATE TABLE empties a table completely. Logically, this is equivalent to a DELETE statement that deletes all rows, but there are practical differences under some circumstances.
For InnoDB, TRUNCATE TABLE is mapped to DELETE, so there is no difference. For other storage engines, TRUNCATE TABLE differs from DELETE FROM ... in the following ways from MySQL 4.0 and up:
Truncate operations drop and re-create the table, which is much faster than deleting rows one by one.
Truncate operations are not transaction-safe; you will get an error if you have an active transaction or an active table lock.
The number of deleted rows is not returned.
As long as the table definition file tbl_name.frm is valid, the table can be re-created as an empty table with TRUNCATE TABLE, even if the data or index files have become corrupted.
The table handler does not remember the last used AUTO_INCREMENT value, but starts counting from the beginning. This is true even for MyISAM, which normally does not reuse sequence values.
In MySQL 3.23, TRUNCATE TABLE is mapped to COMMIT; DELETE FROM tbl_name, so it behaves like DELETE. See DELETE.
TRUNCATE TABLE is an Oracle SQL extension. This statement was added in MySQL 3.23.28, although from 3.23.28 to 3.23.32, the keyword TABLE must be omitted.
UPDATE [LOW_PRIORITY] [IGNORE] tbl_name
SET col_name1=expr1 [, col_name2=expr2 ...]
[WHERE where_definition]
[ORDER BY ...]
[LIMIT row_count]
Multiple-table syntax:
UPDATE [LOW_PRIORITY] [IGNORE] tbl_name [, tbl_name ...]
SET col_name1=expr1 [, col_name2=expr2 ...]
[WHERE where_definition]
The UPDATE statement updates columns in existing table rows with new values. The SET clause indicates which columns to modify and the values they should be given. The WHERE clause, if given, specifies which rows should be updated. Otherwise, all rows are updated. If the ORDER BY clause is specified, the rows will be updated in the order that is specified. The LIMIT clause places a limit on the number of rows that can be updated.
The UPDATE statement supports the following modifiers:
If you specify the LOW_PRIORITY keyword, execution of the UPDATE is delayed until no other clients are reading from the table.
If you specify the IGNORE keyword, the update statement will not abort even if duplicate-key errors occur during the update. Rows for which conflicts occur are not updated.
If you access a column from tbl_name in an expression, UPDATE uses the current value of the column. For example, the following statement sets the age column to one more than its current value:
mysql> UPDATE persondata SET age=age+1;
UPDATE assignments are evaluated from left to right. For example, the following statement doubles the age column, then increments it:
mysql> UPDATE persondata SET age=age*2, age=age+1;
If you set a column to the value it currently has, MySQL notices this and doesn't update it.
If you update a column that has been declared NOT NULL by setting to NULL, the column is set to the default value appropriate for the column type and the warning count is incremented. The default value is 0 for numeric types, the empty string ('') for string types, and the “zero” value for date and time types.
UPDATE returns the number of rows that were actually changed. In MySQL 3.22 or later, the mysql_info() C API function returns the number of rows that were matched and updated and the number of warnings that occurred during the UPDATE.
Starting from MySQL 3.23, you can use LIMIT row_count to restrict the scope of the UPDATE. A LIMIT clause works as follows:
Before MySQL 4.0.13, LIMIT is a rows-affected restriction. The statement stops as soon as it has changed row_count rows that satisfy the WHERE clause.
From 4.0.13 on, LIMIT is a rows-matched restriction. The statement stops as soon as it has found row_count rows that satisfy the WHERE clause, whether or not they actually were changed.
If an UPDATE statement includes an ORDER BY clause, the rows are updated in the order specified by the clause. ORDER BY can be used from MySQL 4.0.0.
Starting with MySQL 4.0.4, you can also perform UPDATE operations that cover multiple tables:
UPDATE items,month SET items.price=month.price WHERE items.id=month.id;
The example shows an inner join using the comma operator, but multiple-table UPDATE statements can use any type of join allowed in SELECT statements, such as LEFT JOIN.
Note: You cannot use ORDER BY or LIMIT with multiple-table UPDATE.
Before MySQL 4.0.18, you need the UPDATE privilege for all tables used in a multiple-table UPDATE, even if they were not updated. As of MySQL 4.0.18, you need only the SELECT privilege for any columns that are read but not modified.
If you use a multiple-table UPDATE statement involving InnoDB tables for which there are foreign key constraints, the MySQL optimizer might process tables in an order that differs from that of their parent/child relationship. In this case, the statement will fail and roll back. Instead, update a single table and rely on the ON UPDATE capabilities that InnoDB provides to cause the other tables to be modified accordingly.
Currently, you cannot update a table and select from the same table in a subquery.