SQL中的CASE WHEN语句是经常要用到的,下面将结合实例,为您详解CASE WHEN语句的使用,供您参考,希望对您学习SQL语句能有所帮助。 一张表数据如下 1900-1-1 胜 1900-1-1 胜 1900-1-1 负 1900-1-2 胜 1900-1-2 胜 写出一条SQL语句,使检索结果如下: 胜 负 19
SQL中的CASE WHEN语句是经常要用到的,下面将结合实例,为您详解CASE WHEN语句的使用,供您参考,希望对您学习SQL语句能有所帮助。
一张表数据如下1900-1-1 胜
1900-1-1 胜
1900-1-1 负
1900-1-2 胜
1900-1-2 胜
写出一条SQL语句,使检索结果如下:
胜 负
1900-1-1 2 1
1900-1-2 2 0
我随手建了这样一个表:
50) null)
并将上面的数据都插入到表中。
经过一番尝试和修改,终于得到了答案:
select distinct Date, Result 1 , Result 1 '负' from test group by date
这里我要说的,其实是SQL中case when的用法。它在普通的SQL语句中似乎并不常见,我本人以前也没在实际项目中使用过。遇到类似问题,往往通过代码或多条SQL语句实现。或者是如下这种丑陋的SQL,并且还伴随着很多潜在的BUG(如,当没有‘负’时)。
select a.date,a.a1 胜,b.b1 负 from datedate) a, (datedate) b where a.date=b.date
我们不妨来复习一下CASE WHEN的语法。
CASE WHEN有两种用法,一种是类似上面例子中那样的简单CASE函数:
CASE result 1 2 ELSE 0 END
还有一种是CASE搜索函数:
1 2 ELSE 0 END
其中result='胜'可以替换为其他条件表达式。如果有多个CASE WHEN表达式符合条件,美国服务器,将只返回第一个符合条件的子句,网站空间,香港虚拟主机,其余子句将被忽略。
用CASE WHEN语句可以简化我们平时工作中遇到的很多问题。如性别在表中存的是数字1、2,但是希望查询出来男、女时,可以这样:

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