13.2.11.2 Comparisons Using Subqueries

The most common use of a subquery is in the form:

Where comparison_operator is one of these operators:

For example:

MySQL also permits this construct:

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 rows in table t1 for which the column1 value is equal to a maximum value in table 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:

For a comparison of the subquery to a scalar, the subquery must return a scalar. For a comparison of the subquery to a row constructor, the subquery must be a row subquery that returns a row with the same number of values as the row constructor. See Section 13.2.11.5, “Row Subqueries”.