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sql语法case用法

程序员文章站 2022-07-12 15:36:17
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关键词: informix,sql,ca                                           

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 < 10 THEN 'Bargain'

WHEN price BETWEEN 10 and 20 THEN 'Average'

ELSE 'Gift to impress relatives'

END

FROM titles

ORDER BY price

GO

对于informix不能用以上形式,可以如下:

USE pubs

GO

SELECT

Title,

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

Price Range

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

对于informix中形式有所不同,例子

select

case when td_acc1_no[1,3]='610' then get_contrast(td_acc1_no)

                 else get_contrast(td_td_acct_no)

                 end sx_acct_no

,               sum(td_actu_amt) sx_bal

from dcc_tdacnacn where td_td_acct_no in ('6107111001014107111000050182',

'6107111001014107111000050168')

group by   1

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。
相关标签: SQL Informix Go