mysql：子查询from&子查询exists

子查询from

查询出来的结果集其实就是一张表，可以把返回的表继续查询

select goods_id,goods_name from (select * from goods order by goods_id) as tmp group by cat_id 

子查询exists

判断查询子句是否有记录，如果有一条或多条记录存在返回 True，否则返回 False。

语法：

SELECT column_name(s)
FROM table_name
WHERE EXISTS
(SELECT column_name FROM table_name WHERE condition);

例子：

// 查出所有商品的栏目
select * from category where exists (select * from goods where goods.cat_id = category.cat_id)