Subqueries are legal in a SELECT statement's FROM clause. The actual syntax is:

SELECT ... FROM (subquery) [AS] name ...

The [AS] name clause is mandatory, because every table in a FROM clause must have a name. Any columns in the subquery select list must have unique names. You can find this syntax described elsewhere in this manual, where the term used is “derived tables.”

For the sake of illustration, assume that you have this table:

CREATE TABLE t1 (s1 INT, s2 CHAR(5), s3 FLOAT);

Here is how to use a subquery in the FROM clause, using the example table:

INSERT INTO t1 VALUES (1,'1',1.0);
INSERT INTO t1 VALUES (2,'2',2.0);
SELECT sb1,sb2,sb3
FROM (SELECT s1 AS sb1, s2 AS sb2, s3*2 AS sb3 FROM t1) AS sb
WHERE sb1 > 1;

Result: 2, '2', 4.0.

Here is another example: Suppose that you want to know the average of a set of sums for a grouped table. This does not work:

SELECT AVG(SUM(column1)) FROM t1 GROUP BY column1;

However, this query provides the desired information:

SELECT AVG(sum_column1)
FROM (SELECT SUM(column1) AS sum_column1
FROM t1 GROUP BY column1) AS t1;

Notice that the column name used within the subquery (sum_column1) is recognized in the outer query.

Subqueries in the FROM clause can return a scalar, column, row, or table. Subqueries in the FROM clause cannot be correlated subqueries.

Subqueries in the FROM clause are executed even for the EXPLAIN statement (that is, derived temporary tables are built). This occurs because upper level queries need information about all tables during optimization phase.