使用SQL Server中的内部联接使用If-Else条件更新行？

if (quantity - @TransferQnty) = 0
   update PD 
   set attrited = 'true'
else 
  update PD 
  set quantity = quantity - @TransferQnty

from K_RT_PurchaseDet as PD
inner join K_RT_Productdetails as PS on PD.product=PS.sno where PS.productname=@Purchaseid
if (quantity - @TransferQnty) = 0
   update PD