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 <</SPAN>myColumnSpec > =
CASE
WHEN
<</SPAN>A > THEN <</SPAN>somethingA >
WHEN
<</SPAN>B > THEN <</SPAN>somethingB >
ELSE
<</SPAN>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 <</SPAN> 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 <</SPAN> 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 <</SPAN> 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 <</SPAN> 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 <</SPAN> 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。

除了选择自定义字段之外,在很多情况下
CASE 都非常有用。再深入一步,你还可以得到你以前认为不可能得到的分组排序结果集。
posted on 2010-09-16 08:38  vibratea  阅读(412)  评论(0编辑  收藏  举报