sql复杂案例

　　工作中往往会遇到非常棘手的数据查询,运营人员不知道你的数据库表是如何设计的,也不知道你的数据库记录了啥数据,他只知道自己需要看什么数据,甚至有些数据根本就不存在.

　　单表查询的难度: 一张数据库的表table,如下图,一个房间两个人对局,房主houser 挑战者challenger,只记录了赢家winner,以及分数point

　　需求,列出  玩家,今天输的总分数,今天赢的总分数,最终输赢的分数最终的查询的结果是

　　暂时不吐槽表的设计者,以及提出需求的运营人员. 要把这个数据查出来,得用很多sql技巧,不限于 case when ,left join ,right join,union

　　来看一个最简单的查询语句:

　　select /*something*/ from table  #从表中查询某字段

　　根据需求我们可以得出初步sql:

　　select player,totallose,totalwin,(totallose+totalwin) from table  #查询玩家输的总数,赢得总数,最终分数,

　　显然从table不能直接找到需求的字段,而是要经过一定的逻辑处理和运算

　　select player,totallose,totalwin,(totallose+totalwin) from (/*something*/)

　　括号里面的(/***/)就是我们需要对数据进行一些逻辑处理

　　第一步:查询输的,把输的人查出来,以及输的总数

　　(select case when winner=houser then challenger when winner=challenger then houser else 0 end AS player,

　　sum(-point) AS lose from table group by player ) AS a

　　第二步,左联一下,把原本输的人的赢得总数查出来

　　LEFT JOIN (SELECT winner AS player ,sum(point) AS win group by player) AS b

　　ON a.player=b.player

　　第三步,把赢的人,和赢的总数查出来,即把刚才的左联变成右连

　　(select case when winner=houser then challenger when winner=challenger then houser else 0 end AS player, sum(-point) AS lose from table group by player ) AS a

　　RIGHT JOIN (SELECT winner AS player ,sum(point) AS win group by player) AS b

　　ON a.player=b.player

　　第四步 把左联和右连的结果再union一下

　　回到 我们一开始的最简单的那条查询

　　select player,totallose,totalwin,(totallose+totalwin) from (/***/)

　　最终,我们要把 (/***/) 完成, 把第一二步查出来的结果作为数据从中查出玩家,再union一下右连接

SELECT

　　a.player AS player , a.lose AS totallose, b.win AS totalwin, (totallose+totalwin) AS total

FROM

　　( SELECT

　　　　case when winner=houser then challenger when  winner=challenger then houser else 0 end AS player, sum(-point)     AS   lose

　　 FROM

　　　　table

　　GROUP BY player

　　)  AS a

LEFT JION

　　(SELECT

　　　　winner AS player , sum(point) AS win

　　FROM

　　　　table

　　GROUP BY

　　　　player) AS b

ON

　　a.player=b.player

UNION

SELECT

　　a.player AS player , a.lose AS totallose, b.win AS totalwin, (totallose+totalwin) AS total

FROM

　　( SELECT

　　　　case when winner=houser then challenger when winner=challenger then houser else 0 end  AS player, sum(-point)     AS   lose

　　 FROM

　　　　table

　　GROUP BY player

　　)  AS a

RIGHT JION

　　(SELECT

　　　　winner AS player ,sum(point) AS win

　　FROM

　　　　table

　　GROUP BY

　　　　player) AS b

ON

　　a.player=b.player

ORDER BY total DESC

本文完......

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