MySql的分组数据(创建分组，分组中带总计，过滤分组数据，分组数据排序，select语句执行顺序)

1.创建分组(group by)

示例：查询一共有多少条数据，按照vend_id的不同来分组显示
select vend_id,count( * ) from products group by vend_id;

2.创建摘要(with rollup)

示例:同上，上面不带总计，需要加上对应关键字
select vend_id,count( * ) from products group by vend_id with rollup;

3.过滤分组(having)类似where的功能，只是在分组中不能使用where

示例：过滤出每个分组条数大于二的数据
根据用户id进行分组，条件是返回分组中有数据条数等于2的
select id,count( * ) from orders group by id having count( * ) = 2;

4.分组和排序

示例：根据订单编号分组，计算出每个订单编号的下的商品总价格，且总价格要大于50，并且按照总价格的大小进行从小到大订单排序；
select order_num,sum( price * quantity ) as ordertotal from orderitems group by order_num having sum( price * quantity ) > 50 order by ordertotal;

5.select 语句的执行顺序

1. select
2. from
3. where
4. group by
5. having
6. order by
7. limit