mysql 自我总结

group by  和  DISTINCT

          group by按组划分，每组只显示1条记录

          distinct后接多个字段，保证每个字段组合的记录唯一（distinct id,name from ）

groupby having 和 where 

         SELECT *,(SUM(win)/COUNT(*)) as present  FROM `xxtable`  GROUP BY hero HAVING present>0.1

SELECT *,(SUM(win)/COUNT(*)) as present  FROM `xxtable` where present>0.1 GROUP BY hero (语句错误，找不到present)

where 子句的作用是在对查询结果进行分组前，将不符合where条件的行去掉，即在分组之前过滤数据，条件中不能包含聚组函数，使用where条件显示特定的行

having用在聚合函数中，判断聚合函数（sum，count，avg，）的值是否大于某个值

实例特训：

1.英雄联盟多玩魔盒

表

A:获取胜率排名前2的英雄和对局

SELECT *,(SUM(win)/COUNT(*)) as present  FROM `aa` WHERE userid=1 GROUP BY hero ORDER BY present desc limit 2

B:获取胜率大于0.5的英雄和对局

SELECT *,(SUM(win)/COUNT(*)) as present  FROM `aa` WHERE userid=1 GROUP BY hero HAVING present>0.5 ORDER BY present desc