A subquery is a
SELECT statement 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 statement), and
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 within parentheses.
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 t1WHERE 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:
LIMIT, joins, index hints,
UNION constructs, 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
UPDATE, and (because subqueries can be used in the
For information about how the optimizer handles subqueries, see Section 8.13.16, "Subquery Optimization". For a discussion of restrictions on subquery use, including performance issues for certain forms of subquery syntax, see Section D.4, "Restrictions on Subqueries".