今天注意到，公司的代码里经常用到exists这个用法。也许个人习惯不一样吧，测试了下性能。对比下，这三种效率上的区别。
mysql> select SQL_NO_CACHE count(*) from test1 where id not in(select id from test2);
+----------+
| count(*) |
+----------+
| 215203 |
+----------+
1 row in set (5.81 sec)
mysql> select SQL_NO_CACHE count(*) from test1 where not exists (select * from test2 where test2.id=test1.id);
+----------+
| count(*) |
+----------+
| 215203 |
+----------+
1 row in set (5.25 sec)
mysql> select SQL_NO_CACHE count(*) from test1 left join test2 on test1.id=test2.id where test2.id is null;
+----------+
| count(*) |
+----------+
| 215203 |
+----------+
1 row in set (4.63 sec)
结论：生产环境里，应尽量避免使用子查询，用left join表连接取代之。
not in,not exists,left join性能对比
猜你喜欢
转载自qieyi28.iteye.com/blog/1725632
今日推荐
周排行