case函数妙用
case函数的妙用select person,ticket_name,sum(case when ticket_name is not null then 1 end) as 总票数
,sum(case when ticket_price is not null then ticket_price end) as 总金额 from ticket_table group by person,ticket_name
是从网上粘贴的,数据就不附上了
将表的数据根据person ticket_name 分组 每个person ticket_name 分到的数据在遍历
case when ticket_name is not null then 1 end 当数据ticket_name is not null 的时候ticket返回的是1,sum就加一;
当后面有where 语句的时候先执行where条件
where -----group by ---- select --------case
仅是自己现阶段的理解
原例http://blog.csdn.net/u011244173/article/details/46118367