Subqueries in a FROM Clause (Derived Tables)

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.

ORACLE mode

Correlation Column List

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 |
+-------------+

From MariaDB 10.6 in ORACLE mode, the following is permitted:

SELECT * FROM (SELECT 1 FROM DUAL), (SELECT 2 FROM DUAL);

From MariaDB 11.7. In this example, the second column of the derived table dt is used both within (WHERE c2 > 0), and outside, (WHERE a2 > 10), the specification. Both conditions apply to t1.c2.

CREATE OR REPLACE TABLE t1(c1 INT, c2 INT, c3 INT);

SELECT a1, a2 FROM (SELECT c1, c2, c3 FROM t1 WHERE c2 > 0) AS dt (a1, a2, a3);