ASA SQL Reference
SQL Language Elements
Search conditions
Subqueries that return exactly one column and either zero or one row can be used in any SQL statement wherever a column name could be used, including in the middle of an expression.
For example, expressions can be compared to subqueries in comparison conditions (see Comparison operators) as long as the subquery does not return more than one row. If the subquery (which must have one column) returns one row, then the value of that row is compared to the expression. If a subquery returns no rows, its value is NULL.
Subqueries that return exactly one column and any number of rows can be used in IN conditions, ANY conditions, and ALL conditions. Subqueries that return any number of columns and rows can be used in EXISTS conditions. These conditions are discussed in the following sections.