exists和not exists对应 in 和not in
不同在于，在select 查询中，exists和in的执行顺序不同。
[sql]
mysql> select * from category;
+----+---------+
| id | c_name |
+----+---------+
| 1 | ios |
| 2 | android |
| 3 | sb |
+----+---------+
3 rows in set
商品表： www.2cto.com
[sql]
mysql> select * from goods;
+----+---------+--------+-------+-----+
| id | name | cat_id | price | num |
+----+---------+--------+-------+-----+
| 1 | 苹果 | 1 | 4999 | 2 |
| 2 | nexus4 | 2 | 1999 | 3 |
| 4 | 荣耀2 | 2 | 1888 | 5 |
| 6 | 三星 | 2 | 3000 | 2 |
+----+---------+--------+-------+-----+
6 rows in se
www.2cto.com
要求：只找出分类下有商品的分类
[sql]
mysql> select * from category c where exists (select * from goods where cat_id=c.id);
+----+---------+
| id | c_name |
+----+---------+
| 1 | ios |
| 2 | android |
+----+---------+
2 rows in set
先执行select查询，结果再和子句中的select结果比较。和in相反。