数据库分组统计

一.分组统计（聚合函数不能用order by进行排序，不能用where进行筛选）
1.group by语句（写在where和order by之间）
select category_id,sum(stock) from item group by category_id;
select avg(price) from item where category_id in (13,11) group by category_id;

2.对分组数据进行筛选，应该在group by中写条件(having)
select category_id,sum(stock) from item group by category_id having sum(stock)>1000;

3.尽量写在where内，效率高
select category_id,sum(stock) from item where category_id>10 ground by category_id;
select category_id,sum(stock) from item group by category_id having category_id>10;

4.分组后排序
select category_id,max(price) from item group by category_id order by category_id desc;
select category_id,sum(price*stock) as s from item group by category_id order by s desc;

二.子查询
1.查询所有有地址的用户的id
select distinct user_id from address;
2.查询所有有地址的用户的全部信息
select*from user where id in (1,3,4);
select*from user where id in (select distinct user_id from address);
3.查询所有有商品的分类的id
select distinct category_id from item;
4.查询所有有商品的分类的全部信息
select*from category where id in (7,8,9);
select*from category where id in (select distinct category_id from item);
5.采用exists也可以实现上述逻辑
select*from user where exists (select*from address where user_id=user.id);
select*from category where exists (select*from item where category_id=category.id);
6.查询出所有用户以及他们所包含的地址数量
select*,(select count(*) from address where user_id=user.id) as amount from user;
7.查询出所有商品分类以及每个分类所包含的库存总数

select*,(select count(*) from item where category_id=category.id) as amount from category;

三.关联查询数据
1.等值连接：查询出A和B相匹配的数据
select*from category,item where category.id=item.category_id;
select u.*,a.name 姓名,a.address from user as u,address as a where u.id=a.user_id;
2.内连接：其作用与等值连接相同
select*from category inner join item on category.id=item.category_id;
select*from category c join item i on c.id=i.category_id order by i.id;
select*from user u join address a on u.id=a.user_id join city c on a.city_id=c.id;

3.左外连接：查询出A的所有数据，以及与A匹配的B的数据

select left(name,1) surname, count(*) from address group by surname;——查询同一姓氏的个数

select*from category left outer join item on category.id=item.category_id;
select*from category left join item on category.id=item.category_id;
4.右外连接：查询出B的所有数据，以及与B匹配的A的数据
select*from category right outer join item on category.id=item.category_id;
select*from category right join item on category.id=item.category_id;