Oracle 5.0, Reference Manual

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Subquery Syntax
1105
mysql> EXPLAIN SELECT f1(5);
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | No tables used |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
1 row in set (0.00 sec)
mysql> SELECT * FROM t2;
Empty set (0.00 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;
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
| 1 | PRIMARY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | 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;
+----+-------------+------------+--------+---------------+------+---------+------+------+---------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+---------------+------+---------+------+------+---------------------+
| 1 | PRIMARY | a1 | system | NULL | NULL | NULL | NULL | 0 | const row not found |
| 1 | PRIMARY | <derived2> | system | NULL | NULL | NULL | NULL | 1 | |
| 2 | DERIVED | NULL | NULL | NULL | NULL | NULL | NULL | NULL | 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()
[957] function is executed once for each row in
t1
:
EXPLAIN SELECT * FROM t1 AS a1, (SELECT BENCHMARK(1000000, MD5(NOW())));
13.2.9.9. Subquery Errors
There are some errors that apply only to subqueries. This section describes them.
• Unsupported subquery syntax:
ERROR 1235 (ER_NOT_SUPPORTED_YET)
SQLSTATE = 42000
Message = "This version of MySQL doesn't yet support
'LIMIT & IN/ALL/ANY/SOME subquery'"