[标准]SQL的case when then 的使用范例

例如,下面的语句显示中文年月  
   
  select   getdate()   as   日期,case   month(getdate())  
  when   11   then   '十一'  
  when   12   then   '十二'  
  else   substring('一二三四五六七八九十',   month(getdate()),1)  
  end+'月'   as   月份

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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，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 <myColumnSpec> =
CASE
WHEN <A> THEN <somethingA>
WHEN <B> THEN <somethingB>
ELSE <somethingE>
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 < 10 THEN 'Bargain'
        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 < 10 THEN 'Bargain'
        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 < 10 THEN 'Bargain'
        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 < 10 THEN 'Bargain'
        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 < 10 THEN 'Bargain'
        WHEN price BETWEEN 10 and 20 THEN 'Average'
        ELSE 'Gift to impress relatives'
    END,
     Title
GO

注意，为了在 GROUP BY 块中使用 CASE，查询语句需要在 GROUP BY 块中重复 SELECT 块中的 CASE 块。