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MySQL 5.5 Reference Manual  /  ...  /  Derived Tables (Subqueries in the FROM Clause)

13.2.10.8 Derived Tables (Subqueries in the FROM Clause)

A derived table is a subquery in a SELECT statement FROM clause:

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

The [AS] tbl_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.

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.

Derived tables can return a scalar, column, row, or table.

Derived tables cannot be correlated subqueries, or contain outer references or references to other tables of the same SELECT.

Subqueries in the FROM clause are executed even for the EXPLAIN statement (that is, derived temporary tables are materialized). This occurs because upper-level queries need information about all tables during the optimization phase, and the table represented by a subquery in the FROM clause is unavailable unless the subquery is executed.

It is possible under certain circumstances that using EXPLAIN SELECT will modify table data. This can occur if the outer query accesses any tables and an inner query invokes a stored function that changes one or more rows of a table. Suppose that there are two tables t1 and t2 in database d1, and a stored function f1 that modifies t2, created as shown here:

CREATE DATABASE d1;
USE d1;
CREATE TABLE t1 (c1 INT);
CREATE TABLE t2 (c1 INT);
CREATE FUNCTION f1(p1 INT) RETURNS INT
  BEGIN
    INSERT INTO t2 VALUES (p1);
    RETURN p1;
  END;

Referencing the function directly in an EXPLAIN SELECT has no effect on t2, as shown here:

mysql> SELECT * FROM t2;
Empty set (0.02 sec)

mysql> EXPLAIN SELECT f1(5)\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: NULL
         type: NULL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: NULL
        Extra: No tables used
1 row in set (0.01 sec)

mysql> SELECT * FROM t2;
Empty set (0.01 sec)

This is because the SELECT statement did not reference any tables, as can be seen in the table and Extra columns of the output. This is also true of the following nested SELECT:

mysql> EXPLAIN SELECT NOW() AS a1, (SELECT f1(5)) AS a2\G
*************************** 1. row ***************************
           id: 1
  select_type: PRIMARY
        table: NULL
         type: NULL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: NULL
     filtered: NULL
        Extra: No tables used
1 row in set, 1 warning (0.00 sec)

mysql> SHOW WARNINGS;
+-------+------+------------------------------------------+
| Level | Code | Message                                  |
+-------+------+------------------------------------------+
| Note  | 1249 | Select 2 was reduced during optimization |
+-------+------+------------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT * FROM t2;
Empty set (0.00 sec)

However, if the outer SELECT references any tables, the optimizer executes the statement in the subquery as well:

mysql> EXPLAIN SELECT * FROM t1 AS a1, (SELECT f1(5)) AS a2\G
*************************** 1. row ***************************
           id: 1
  select_type: PRIMARY
        table: <derived2>
         type: system
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 1
        Extra: 
*************************** 2. row ***************************
           id: 1
  select_type: PRIMARY
        table: a1
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 1
        Extra: 
*************************** 3. row ***************************
           id: 2
  select_type: DERIVED
        table: NULL
         type: NULL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: NULL
        Extra: No tables used
3 rows in set (0.00 sec)

mysql> SELECT * FROM t2;
+------+
| c1   |
+------+
|    5 |
+------+
1 row in set (0.00 sec)

This also means that an EXPLAIN SELECT statement such as the one shown here may take a long time to execute because the BENCHMARK() function is executed once for each row in t1:

EXPLAIN SELECT * FROM t1 AS a1, (SELECT BENCHMARK(1000000, MD5(NOW())));

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