1select * from product join category;  -- 结果为笛卡尔积 8条记录
 2
 3select * from product p join category c on p.category_id=c.category_id; --  按照id相等去连接， 忽略id 为空的记录  内连接 null的数据不会显示
 4
 5
 6select * from product p left join category c on p.category_id=c.category_id; -- 左外连接， 左边为主表
 7
 8
 9select p.category_id,category_name ,count(1) as total from product p left join category c on p.category_id=c.category_id group by p.category_id; -- count 聚合函数
10
11select p.category_id,category_name ,MIN(price) as total from product p left join category c on p.category_id=c.category_id group by p.category_id; -- min 聚合函数
12
13
14select * from product p left join (
15    select p.category_id,category_name ,MIN(price) as min_price
16    from product p left join category c
17        on p.category_id=c.category_id group by p.category_id) as cat_min
18on p.category_id = cat_min.category_id
19where p.price = cat_min.min_price;
20/*
21 获取某个品类中价格最低的商品
22 */