holdlock applies a read-lock on the data until the transaction ends.    holdlock -makes a shared lock on a specified table or view more restrictive 
by holding it until the completion of a transaction (instead of releasing the 
shared lock as soon as the required data page is no longer needed, whether or 
not the transaction has been completed).    使用時機 : 如計算庫存總量時, 限制計算當時之入/出庫資料交易.    Example Transcat-SQL :
begin
  declare @total_QTY float 
  begin tran
     select @total_QTY=sum(QTY) from Inventory holdlock 
     ...
  commit tran
end