请问这语句该怎么写？

Question

--楼主，你先看看这个等于多少？？
select datediff(hour,qssj,zzsj) + datediff(hour,ycqssj,yczzsj) from wjga_pwgzb

Answer 1

select Count(*)
from
(
select (datediff(hour,qssj,zzsj) + datediff(hour,ycqssj,yczzsj)) as AAA from wjga_pwgzb
) a 
where a.AAA>2借用aw511(点点星灯) 的，应该还可以用Case用法

Answer 2

测试select datediff(hour,GetDate(),null)
结果为Null  
楼主要用IsNull函数select 
Sum(
case when (datediff(hour,qssj,zzsj)+ datediff(hour,IsNull(ycqssj,'1900-01-01'),IsNull(yczzsj,'1900-01-01')))>2 then
 1 else 0 end
) as AAAAselect Count(*)
from
(
select (datediff(hour,qssj,zzsj) + datediff(hour,IsNull(ycqssj,'1900-01-01'),IsNull(yczzsj,'1900-01-01'))) as AAA from wjga_pwgzb
) a 
where a.AAA>2from wjga_pwgzb