MySQL not in嵌套查询改写成外连接方式
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：https://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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