sql 查询某个字段为空的时候，更改where 条件 如何实现
select
case
when ContractEndTime=''
then
(select ContractName from dbo.ContractExecution where ContractBeginTime <'2013-10-1 ')
else
(select ContractName from ContractExecution where ContractBeginTime <'2013-10-1 ' and ContractEndTime>'2012-10-1' )
end
from dbo.ContractExecution
--where
order by ID desc
--消息 512，级别 16，状态 1，第 1 行
-子查询返回的值不止一个。当子查询跟随在 =、!=、<、<=、>、>= 之后，或子查询用作表达式时，这种情况是不允许的。

[解决办法]

SQL code

SELECT  ContractName FROM    dbo.ContractExecution WHERE   ContractBeginTime < CASE WHEN ContractEndTime = '' THEN '2013-10-1 '                                  ELSE '2013-10-1 '                             END         AND ContractEndTime > CASE WHEN ContractEndTime = '' THEN 0                                    ELSE '2012-10-1'                               END ORDER BY ID DESC
[解决办法]
1.select ContractName from ContractExecution where ContractBeginTime <'2013-10-1 ' and ContractEndTime>case when ContractEndTime='' then '1900-1-1' else '2012-10-1' end  

2.if ContractEndTime=''
select ContractName from dbo.ContractExecution where ContractBeginTime <'2013-10-1 '
else
select ContractName from ContractExecution where ContractBeginTime <'2013-10-1 ' and ContractEndTime>'2012-10-1'  
以上两种方法都可以实现楼主想要的。