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sql语句判断方法之一 Case具有两种格式。简单Case函数和Case搜索函数。 --简单Case函数 CASE sex WHEN '1' THEN '男' WHEN '2' THEN '女' ELSE '其他' END --Case搜索函数 CASE WHEN sex = '1' THEN '男' WHEN sex = '2' THEN '女' ELSE '其他' END 这两种方

sql语句判断方法之一
Case具有两种格式。简单Case函数和Case搜索函数。
--简单Case函数
CASE sex
WHEN '1' THEN '男'
WHEN '2' THEN '女'
ELSE '其他' END
--Case搜索函数
CASE WHEN sex = '1' THEN '男'
WHEN sex = '2' THEN '女'
ELSE '其他' END


这两种方式,可以实现相同的功能。简单Case函数的写法相对比较简洁,但是和Case搜索函数相比,功能方面会有些限制,比如写判断式。
还有一个需要注意的问题,Case函数只返回第一个符合条件的值,剩下的Case部分将会被自动忽略。

例子:
有一张表,里面有3个字段:语文,数学,免备案空间,英语。其中有3条记录分别表示语文70分,数学80分,英语58分,请用一条sql语句查询出这三条记录并按以下条件显示出来(并写出您的思路):
大于或等于80表示优秀,美国服务器,大于或等于60表示及格,小于60分表示不及格。
显示格式:
语文 数学 英语
及格 优秀 不及格
------------------------------------------
select
(case when 语文>=80 then '优秀'
when 语文>=60 then '及格'
else '不及格') as 语文,
(case when 数学>=80 then '优秀'
when 数学>=60 then '及格'
else '不及格') as 数学,
(case when 英语>=80 then '优秀'
when 英语>=60 then '及格'
else '不及格') as 英语,
from table


CASE 可能是 SQL 中被误用最多的关键字之一。虽然你可能以前用过这个关键字来创建字段,但是它还具有更多用法。例如,美国服务器,你可以在 WHERE 子句中使用 CASE。

首先让我们看一下 CASE 的语法。在一般的 SELECT 中,其语法如下:

SELECT =
            CASE
            WHEN THEN
            WHEN THEN
            ELSE
            END


在上面的代码中需要用具体的参数代替尖括号中的内容。下面是一个简单的例子:

USE pubs
            GO
            SELECT
            Title,
            'Price Range' =
            CASE
            WHEN price IS NULL THEN 'Unpriced'
            WHEN price             WHEN price BETWEEN 10 and 20 THEN 'Average'
            ELSE 'Gift to impress relatives'
            END
            FROM titles
            ORDER BY price
            GO


这是 CASE 的典型用法,但是使用 CASE 其实可以做更多的事情。比方说下面的 GROUP BY 子句中的 CASE:

SELECT 'Number of Titles', Count(*)
            FROM titles
            GROUP BY
            CASE
            WHEN price IS NULL THEN 'Unpriced'
            WHEN price             WHEN price BETWEEN 10 and 20 THEN 'Average'
            ELSE 'Gift to impress relatives'
            END
            GO


你甚至还可以组合这些选项,添加一个 ORDER BY 子句,如下所示:

USE pubs
            GO
            SELECT
            CASE
            WHEN price IS NULL THEN 'Unpriced'
            WHEN price             WHEN price BETWEEN 10 and 20 THEN 'Average'
            ELSE 'Gift to impress relatives'
            END AS Range,
            Title
            FROM titles
            GROUP BY
            CASE
            WHEN price IS NULL THEN 'Unpriced'
            WHEN price             WHEN price BETWEEN 10 and 20 THEN 'Average'
            ELSE 'Gift to impress relatives'
            END,
            Title
            ORDER BY
            CASE
            WHEN price IS NULL THEN 'Unpriced'
            WHEN price             WHEN price BETWEEN 10 and 20 THEN 'Average'
            ELSE 'Gift to impress relatives'
            END,
            Title
            GO

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