mysql：子查詢from&子查詢exists

子查詢from

查詢出來的結果集其實就是一張表，可以把返回的表繼續查詢

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

子查詢exists

判斷查詢子句是否有記錄，如果有一條或多條記錄存在返回 True，否則返回 False。

語法：

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

例子：

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