首先让我们看一下 CASE 的语法。在一般的 SELECT 中,其语法如下:<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />
SELECT <myColumnSpec> = CASE <column> WHEN <A> THEN <somethingA> WHEN <B> THEN <somethingB> ELSE <somethingE> END |
在pubs数据库的authors表中,根据作者来自州的名字,打印作者的姓名和州的全名:
USE pubs
GO
SELECT '作者'=
CASE state
WHEN 'CA' THEN '加利弗里亚'
WHEN 'KS' THEN '堪萨斯'
WHEN 'TN' THEN '田纳西'
ELSE '其他州'
END,
au_fname+'.'+au_lname AS au_name
FROM authors
ORDER BY state
GO GO |
上面需要column= A<字段值>,下面的<A>指的是一个表达式,这样灵活性更强:
SELECT <myColumnSpec> = CASE WHEN <A> THEN <somethingA> WHEN <B> THEN <somethingB> ELSE <somethingE> END |
在pubs数据库的titles表中,根据不同价格范围,打印价格状况(自定义字段):
USE pubs GO SELECT Title, 'Price Range' = CASE WHEN price IS NULL THEN '没有定价' WHEN price < 10 THEN '价格偏低' WHEN price BETWEEN 10 and 20 THEN '价格一般' ELSE '价格过高' END FROM titles ORDER BY price GO |
这都是 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。
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。