工作中往往會遇到非常棘手的數據查詢,運營人員不知道你的數據庫表是如何設計的,也不知道你的數據庫記錄了啥數據,他只知道自己需要看什么數據,甚至有些數據根本就不存在.
單表查詢的難度: 一張數據庫的表table,如下圖,一個房間兩個人對局,房主houser 挑戰者challenger,只記錄了贏家winner,以及分數point
需求,列出 玩家,今天輸的總分數,今天贏的總分數,最終輸贏的分數最終的查詢的結果是
暫時不吐槽表的設計者,以及提出需求的運營人員. 要把這個數據查出來,得用很多sql技巧,不限於 case when ,left join ,right join,union
來看一個最簡單的查詢語句:
select /*something*/ from table #從表中查詢某字段
根據需求我們可以得出初步sql:
select player,totallose,totalwin,(totallose+totalwin) from table #查詢玩家輸的總數,贏得總數,最終分數,
顯然從table不能直接找到需求的字段,而是要經過一定的邏輯處理和運算
select player,totallose,totalwin,(totallose+totalwin) from (/*something*/)
括號里面的(/***/)就是我們需要對數據進行一些邏輯處理
第一步:查詢輸的,把輸的人查出來,以及輸的總數
(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,左聯一下,把原本輸的人的贏得總數查出來
LEFT JOIN (SELECT winner AS player ,sum(point) AS win group by player) AS b
ON a.player=b.player
第三步,把贏的人,和贏的總數查出來,即把剛才的左聯變成右連
(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 把左聯和右連的結果再union一下
回到 我們一開始的最簡單的那條查詢
select player,totallose,totalwin,(totallose+totalwin) from (/***/)
最終,我們要把 (/***/) 完成, 把第一二步查出來的結果作為數據從中查出玩家,再union一下右連接
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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