SELECT语句中执行条件逻辑(返回两个日期之间的天数)

case when...then...
else...
end...

[sql] 
SELECT       
 case when hdrq IS null then cast((datediff(day,yqsj,GETDATE()))as int)   
    else cast((datediff(day,yqsj,hdrq))as int)   
 end as 天数  
FROM         hjzx  
ORDER BY id DESC