sql计算两个时间之间的差，并用时分秒表示

这是自己写的方法，总觉得会有更好的办法实现这个效果呢？

SELECT 
case when CONVERT(nvarchar,DATEDIFF(SECOND,FinishAt,getdate() ))>86400  then
CONVERT(nvarchar, DATEDIFF(SECOND,FinishAt,getdate() )/86400)+\'天\'+CONVERT(nvarchar, DATEDIFF(SECOND,FinishAt,getdate() )%86400/3600)+\'时\'+CONVERT(nvarchar, DATEDIFF(SECOND,FinishAt,getdate() )%86400%3600/60)+\'分\'+CONVERT(nvarchar, DATEDIFF(SECOND,FinishAt,getdate() )%86400%3600%60)+\'秒\'
when CONVERT(nvarchar,DATEDIFF(SECOND,FinishAt,getdate() ))>3600 then 
CONVERT(nvarchar, DATEDIFF(SECOND,FinishAt,getdate() )/3600)+\'时\'+CONVERT(nvarchar, DATEDIFF(SECOND,FinishAt,getdate() )%3600/60)+\'分\'+CONVERT(nvarchar, DATEDIFF(SECOND,FinishAt,getdate() )%3600%60)+\'秒\'
when  CONVERT(nvarchar,DATEDIFF(SECOND,FinishAt,getdate() ))>60 then
CONVERT(nvarchar, DATEDIFF(SECOND,FinishAt,getdate() )/60)+\'分\'+CONVERT(nvarchar, DATEDIFF(SECOND,FinishAt,getdate() )%60)+\'秒\'
else CONVERT(nvarchar,DATEDIFF(SECOND,FinishAt,getdate() ))+\'秒\' end TimeSpend   
  FROM [BPMDB].[dbo].[BPMInstTasks] A