select case when where having用法

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

example one:

select c.* from (select t.name, sum(t.fenshu) total, avg(t.fenshu) perscore, count(*) courses from (select name, kechen, (case when fenshu<'80' then '0' else fenshu end) fenshu from course) t group by name) c where c.perscore >= 80

example one data structure:
name kechen fenshu
test 语文    81
test 数学    83
test 英语    92

example two:

select t.* from (select name, (case when yuwen IS NULL then '80' else yuwen end) as yuwen, (case when shuxue IS NULL then '80' else shuxue end) as shuxue, (case when yingyu IS NULL then '80' else yingyu end) as yingyu from score) t where t.yuwen>='80' and t.shuxue>='80' and t.yingyu>='80'

example two data structure:
name yuwen shuxue yingyu
test 81     97    87

example three:

select
	reg.NAME,
	reg.END_TIME,
	reg.DURATION,
	(case reg.status  
		when 'COMPLETE' then 1 
		else 0 
	end)  as complete,
	(case reg.status  
		when 'INCOMPLETE' then 1 
		else 0 
	end)  as incomplete,
	(case reg.status  
		when 'ERROR' then 1 
		else 0 
	end)  as error,
	(case reg.status  
		when 'TIMEOUT' then 1 
		else 0 
	end)  as timeout,
	(case reg.status  
		when 'INFLIGHT' then 1 
		when 'ORPHANED' then 1 
		when 'PENDING' then 1 
		when 'RECONCILE' then 1 
		else 0 
	end)  as inflight  
from
	TM_TRANSACTION_REGISTRY reg

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

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 

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。

“Where” 是一个约束声明，使用Where来约束来之数据库的数据，Where是在结果返回之前起作用的。

“Having”是一个过滤声明，当查询返回结果集以后对查询结果进行过滤操作。