The MERGE storage engine was introduced in MySQL 3.23.25. It is also known as the MRG_MyISAM engine. The code is now reasonably stable.

A MERGE table is a collection of identical MyISAM tables that can be used as one. “Identical” means that all tables have identical column and index information. You can't merge tables in which the columns are listed in a different order, don't have exactly the same columns, or have the indexes in different order. However, any or all of the tables can be compressed with myisampack. See myisampack. Differences in table options such as AVG_ROW_LENGTH, MAX_ROWS, or PACK_KEYS do not matter.

When you create a MERGE table, MySQL creates two files on disk. The files have names that begin with the table name and have an extension to indicate the file type. An .frm file stores the table definition, and an .MRG file contains the names of the tables that should be used as one. (Originally, all used tables had to be in the same database as the MERGE table itself. This restriction has been lifted as of MySQL 4.1.1.)

You can use SELECT, DELETE, UPDATE, and (as of MySQL 4.0) INSERT on the collection of tables. For the moment, you must have SELECT, UPDATE, and DELETE privileges on the tables that you map to a MERGE table.

If you DROP the MERGE table, you are dropping only the MERGE specification. The underlying tables are not affected.

When you create a MERGE table, you must specify a UNION=(list-of-tables) clause that indicates which tables you want to use as one. You can optionally specify an INSERT_METHOD option if you want inserts for the MERGE table to happen in the first or last table of the UNION list. If you don't specify any INSERT_METHOD option or specify it with a value of NO, attempts to insert records into the MERGE table result in an error.

The following example shows how to create a MERGE table:

mysql> CREATE TABLE t1 (
    ->    a INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    ->    message CHAR(20));
mysql> CREATE TABLE t2 (
    ->    a INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    ->    message CHAR(20));
mysql> INSERT INTO t1 (message) VALUES ('Testing'),('table'),('t1');
mysql> INSERT INTO t2 (message) VALUES ('Testing'),('table'),('t2');
mysql> CREATE TABLE total (
    ->    a INT NOT NULL AUTO_INCREMENT,
    ->    message CHAR(20), INDEX(a))
    ->    TYPE=MERGE UNION=(t1,t2) INSERT_METHOD=LAST;

Note that the a column is indexed in the MERGE table, but is not declared as a PRIMARY KEY as it is in the underlying MyISAM tables. This is necessary because a MERGE table cannot enforce uniqueness over the set of underlying tables.

After creating the MERGE table, you can do things like this:

mysql> SELECT * FROM total;
+---+---------+
| a | message |
+---+---------+
| 1 | Testing |
| 2 | table   |
| 3 | t1      |
| 1 | Testing |
| 2 | table   |
| 3 | t2      |
+---+---------+

Note that you can also manipulate the .MRG file directly from outside of the MySQL server:

shell> cd /mysql-data-directory/current-database
shell> ls -1 t1 t2 > total.MRG
shell> mysqladmin flush-tables

To remap a MERGE table to a different collection of MyISAM tables, you can do one of the following:

MERGE tables can help you solve the following problems:

The disadvantages of MERGE tables are: