8_查询之group by.txt

select 5种子句：

where 条件查询

group by 分组

having 筛选

order by 排序

limit 限制结果条数

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group by

group by 用于统计，

一般和max min sum avg count配合使用

max求最大

min求最小

sum求总和

avg求平均

count求总行数

注意：sum是求总和，而count是求总行数。

1.max

取出本店商品中的shop_price最大值:

seclect max(shop_price) from goods;

取出每个栏目中的shop_price最大值:

seclect cat_id,max(shop_price) from goods group by cat_id;

2.min

取出每个栏目中的shop_price最小值:

seclect cat_id,min(shop_price) from goods group by cat_id;

3.sum

取出每个栏目中的shop_price总和:

seclect cat_id,sum(shop_price) from goods group by cat_id;

4.avg

取出每个栏目中的shop_price平均值:

seclect cat_id,avg(shop_price) from goods group by cat_id;

5.count

取出每个栏目中的商品种类:

seclect cat_id,count(*) from goods group by cat_id;

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注意：要把列名当成变量名来看

本店每个商品价格比市场价格低多少钱：

seclect goods_name,market_price - shop_price from goods;

查询每个栏目下面积压的货款：

select cat_id,sum(shop_price * goods_number) from goods group by cat_id;

查询每个栏目下面积压的货款，变量也可以用别名，as ，如(sum(shop_price * goods_number) as huokuan)：

select cat_id,sum(shop_price * goods_number) as huokuan from goods group by cat_id;