Although subqueries are more commonly placed in a WHERE clause, they can also form part of the FROM clause. Such subqueries are commonly called derived tables.

If a subquery is used in this way, you must also use an AS clause to name the result of the subquery.

Examples

CREATE TABLE student (name CHAR(10), test CHAR(10), score TINYINT); 

INSERT INTO student VALUES 
  ('Chun', 'SQL', 75), ('Chun', 'Tuning', 73), 
  ('Esben', 'SQL', 43), ('Esben', 'Tuning', 31), 
  ('Kaolin', 'SQL', 56), ('Kaolin', 'Tuning', 88), 
  ('Tatiana', 'SQL', 87), ('Tatiana', 'Tuning', 83);

Assume that, given the data above, you want to return the average total for all students. In other words, the average of Chun's 148 (75+73), Esben's 74 (43+31), etc.

You cannot do the following:

SELECT AVG(SUM(score)) FROM student GROUP BY name;
ERROR 1111 (HY000): Invalid use of group function

A subquery in the FROM clause is however permitted:

SELECT AVG(sq_sum) FROM (SELECT SUM(score) AS sq_sum FROM student GROUP BY name) AS t;
+-------------+
| AVG(sq_sum) |
+-------------+
|    134.0000 |
+-------------+