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dependent union： union中的第二个或随后的select查询，依赖于外部查询的结果集
- mysql> explain select * from test where id in (select id from test where id = 1000 union all select id from test2) ;
- +----+--------------------+------------+--------+---------------+---------+---------+-------+-------+-----------------+
- | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
- +----+--------------------+------------+--------+---------------+---------+---------+-------+-------+-----------------+
- | 1 | PRIMARY | test | ALL | NULL | NULL | NULL | NULL | 68505 | Using where |
- | 2 | DEPENDENT SUBQUERY | test | const | PRIMARY | PRIMARY | 8 | const | 1 | Using index |
- | 3 | DEPENDENT UNION | test2 | eq_ref | PRIMARY | PRIMARY | 8 | func | 1 | Using index |
- | NULL | UNION RESULT | <union2,3> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
- +----+--------------------+------------+--------+---------------+---------+---------+-------+-------+-----------------+
subquery： 子查询中的第一个select查询，不依赖与外部查询的结果集
- mysql> explain select * from test where id = (select id from test where id = 1000);
- +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+
- | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
- +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+
- | 1 | PRIMARY | test | const | PRIMARY | PRIMARY | 8 | const | 1 | NULL |
- | 2 | SUBQUERY | test | const | PRIMARY | PRIMARY | 8 | const | 1 | Using index |
- +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+
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