在select语句中使用case条件

Question

我想使用案例条件来获取日期之间的值。

我有一个表名用户和一个列日期，我想使用日期条件从数据库中获取值。

select sum(visitor) AS V FROM user  WHERE date  < '21-05-2012' AND dDateTime > '11-05-2012' 
select sum(visitor) AS c FROM user  WHERE date  < '21-06-2012' AND dDateTime > '11-06-2012'
select sum(visitor) AS s FROM user  WHERE date  < '21-07-2012' AND dDateTime > '11-07-2012'
select sum(visitor) AS g FROM user  WHERE date  < '21-08-2012' AND dDateTime > '11-08-2012'

如何在一个语句中完成整个查询。

Answer 1

以下是sum内有case的示例：

select sum(case when date < '21-05-2012' AND dDateTime > '11-05-2012' then visitor end) v 
,      sum(case when date < '21-06-2012' AND dDateTime > '11-06-2012' then visitor end) c
,      sum(case when date < '21-07-2012' AND dDateTime > '11-07-2012' then visitor end) s
,      sum(case when date < '21-08-2012' AND dDateTime > '11-08-2012' then visitor end) g
from   user