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在MySQL中，not in 嵌套查询会在数据库里面创建一张临时表，导致执行效率很低，可以改成外连接的方式处理，效率会好很多。
not in方式
mysql> select * from dept where deptno not in (select deptno from emp); +--------+------------+---------+ | deptno | dname | loc | +--------+------------+---------+ | 40 | OPERATIONS | BOSTON | | 50 | Research | BeiJing | +--------+------------+---------+ 2 rows in set (0.00 sec) mysql> explain select * from dept where deptno not in (select deptno from emp); +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | 1 | PRIMARY | dept | ALL | NULL | NULL | NULL | NULL | 5 | Using where | | 2 | SUBQUERY | emp | ALL | NULL | NULL | NULL | NULL | 14 | NULL | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ 2 rows in set (0.00 sec) 外连接方式
mysql> select * from dept e left join emp d on e.deptno=d.deptno where d.deptno is null; +--------+------------+---------+-------+-------+------+------+----------+------+------+--------+ | deptno | dname | loc | empno | ename | job | mgr | hiredate | sal | com | deptno | +--------+------------+---------+-------+-------+------+------+----------+------+------+--------+ | 40 | OPERATIONS | BOSTON | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | | 50 | Research | BeiJing | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | +--------+------------+---------+-------+-------+------+------+----------+------+------+--------+ 2 rows in set (0.00 sec) mysql> explain select * from dept e left join emp d on e.deptno=d.deptno where d.deptno is null; +----+-------------+-------+------+---------------+------+---------+------+------+----------------------------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+----------------------------------------------------+ | 1 | SIMPLE | e | ALL | NULL | NULL | NULL | NULL | 5 | NULL | | 1 | SIMPLE | d | ALL | NULL | NULL | NULL | NULL | 14 | Using where; Using join buffer (Block Nested Loop) | +----+-------------+-------+------+---------------+------+---------+------+------+----------------------------------------------------+ 2 rows in set (0.00 sec) 来自 “ ITPUB博客 ” ，链接：http://blog.itpub.net/26506993/viewspace-2123663/，如需转载，请注明出处，否则将追究法律责任。
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