A subquery is a
within 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.
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
(SELECT column1 FROM
t2) is the subquery. We say that
the subquery is nested within the outer
query, and in fact it is possible to nest subqueries within other
subqueries, to a considerable depth. A subquery must always appear
The main advantages of subqueries are:
They allow queries that are structured so that it is possible to isolate each part of a statement.
They provide alternative ways to perform operations that would otherwise require complex joins and unions.
Many people find subqueries more readable than complex joins or unions. 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)));
A subquery can return a scalar (a single value), a single row, a single column, or a table (one or more rows of one or more columns). These are called scalar, column, row, and table subqueries. Subqueries that return a particular kind of result often can be used only in certain contexts, as described in the following sections.
There are few restrictions on the type of statements in which
subqueries can be used. A subquery can contain many of the
keywords or clauses that an ordinary
SELECT can contain:
comments, functions, and so on.
In MySQL, you cannot modify a table and select from the same table
in a subquery. This applies to statements such as
UPDATE, and (because subqueries can
be used in the
For information about how the optimizer handles subqueries, see
Section 184.108.40.206, “Optimizing Subqueries with
EXISTS Strategy”. For a
discussion of restrictions on subquery use, including performance
issues for certain forms of subquery syntax, see
Section D.3, “Restrictions on Subqueries”.