mysql 之 exists子查询

子查询可以分为：from where 和 exists子查询

分类表：

mysql> select * from category;+----+---------+| id | c_name  |+----+---------+|  1 | ios     ||  2 | android ||  3 | sb      |+----+---------+3 rows in set

商品表：

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

要求：只找出分类下有商品的分类

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