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 =
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 < THEN Bargain
WHEN price BETWEEN and 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 < THEN Bargain
WHEN price BETWEEN and 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 < THEN Bargain
WHEN price BETWEEN and THEN Average
ELSE Gift to impress relatives
END AS Range
Title
FROM titles
GROUP BY
CASE
WHEN price IS NULL THEN Unpriced
WHEN price < THEN Bargain
WHEN price BETWEEN and THEN Average
ELSE Gift to impress relatives
END
Title
ORDER BY
CASE
WHEN price IS NULL THEN Unpriced
WHEN price < THEN Bargain
WHEN price BETWEEN and 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
