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
1<mycolumnspec> =
2CASE
3WHEN <a> THEN <somethinga>
4WHEN <b> THEN <somethingb>
5ELSE <somethinge>
6END
7
8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:
9
10USE pubs
11GO
12SELECT
13Title,
14'Price Range' =
15CASE
16WHEN price IS NULL THEN 'Unpriced'
17WHEN price < 10 THEN 'Bargain'
18WHEN price BETWEEN 10 and 20 THEN 'Average'
19ELSE 'Gift to impress relatives'
20END
21FROM titles
22ORDER BY price
23GO
24
25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:
26
27SELECT 'Number of Titles', Count(*)
28FROM titles
29GROUP BY
30CASE
31WHEN price IS NULL THEN 'Unpriced'
32WHEN price < 10 THEN 'Bargain'
33WHEN price BETWEEN 10 and 20 THEN 'Average'
34ELSE 'Gift to impress relatives'
35END
36GO
37
38你甚至还可以组合这些选项,添加一个 ORDER BY 子句,如下所示:
39
40USE pubs
41GO
42SELECT
43CASE
44WHEN price IS NULL THEN 'Unpriced'
45WHEN price < 10 THEN 'Bargain'
46WHEN price BETWEEN 10 and 20 THEN 'Average'
47ELSE 'Gift to impress relatives'
48END AS Range,
49Title
50FROM titles
51GROUP BY
52CASE
53WHEN price IS NULL THEN 'Unpriced'
54WHEN price < 10 THEN 'Bargain'
55WHEN price BETWEEN 10 and 20 THEN 'Average'
56ELSE 'Gift to impress relatives'
57END,
58Title
59ORDER BY
60CASE
61WHEN price IS NULL THEN 'Unpriced'
62WHEN price < 10 THEN 'Bargain'
63WHEN price BETWEEN 10 and 20 THEN 'Average'
64ELSE 'Gift to impress relatives'
65END,
66Title
67GO
68
69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。
70
71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。</somethinge></somethingb></b></somethinga></a></mycolumnspec>