CREATE proc test 
@param1 smallint ,
@param2 varchar(14),
@param3 smallint,
@param4 Varchar(50)
as
begin
if  
(select * from comProd 
where flag=isnull(@param1,'')and billNO=isnull(@param2,'')and RowNO=isnull(@param3,'') =1)
then  

 update comProd set UDEF1= @param3 where where flag=isnull(@param1,'')and billNO=isnull(@param2,'')and RowNO=isnull(@param4,'')
else
return
end
GO

------解决方案--------------------

SQL code

create proc test @param1 smallint, @param2 varchar(14), @param3 smallint, @param4 Varchar(50)
as
begin
if ((select count(1) from comProd where flag=isnull(@param1,'')and billNO=isnull(@param2,'')and RowNO=isnull(@param3,''))=1)
  update comProd 
  set UDEF1= @param3 
  where flag=isnull(@param1,'')and billNO=isnull(@param2,'')and RowNO=isnull(@param4,'')
end