统计sql语句改造 (当sign_date不为空时，用sign_date升序排列，当sign_date为空时， 用issu_date升序排列)

select * from (
select '2011-09-14' as sign_date,'2011-09-01' as issu_date
union
select '2011-09-10' as sign_date,'2011-09-05' as issu_date
union
select '2011-09-12' as sign_date,'2011-09-02' as issu_date
union
select '' as sign_date,'2011-09-04' as issu_date
union
select '' as sign_date,'2011-09-05' as issu_date
) a 
order by sign_date ,issu_date

 结果:

结果出来后，发现第一列数据总是空排在前面， 所以在原有的基础上构造NULL让其参与排序。

select case when sign_date='' then'NULL' else sign_date end sign_date,issu_date from (
select '2011-09-14' as sign_date,'2011-09-01' as issu_date
union
select '2011-09-10' as sign_date,'2011-09-05' as issu_date
union
select '2011-09-12' as sign_date,'2011-09-02' as issu_date
union
select '' as sign_date,'2011-09-04' as issu_date
union
select '' as sign_date,'2011-09-05' as issu_date
) a 
order by sign_date ,issu_date

结果: