sql计算两个时间之间的差，并用时分秒表示

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