Which one is better way to insert data? I have two cases 


case

create  proc    createEmployee
    @firstName varchar(255),
    @lastName varchar(255) = null
as
insert into Employee_Tbl values (@firstName, @lastName)

case

create  proc    createEmployee
    @firstName varchar(255),
    @lastName varchar(255) = null
as
insert into Employee_Tbl(FirstName) values (@firstName)

declare @id as int
set @id = SCOPE_IDENTITY()
if @lastName is not null
begin
   update Employee_Tbl
       set LastName  = @lastName 
    where EmpId = @id
end



Of the two code blocks above, which one performs better?
First of all , The stored procedures you have are totally different. So the performance are not really compare-able. 

Thought the final output you receive is the same , in the first SP you are doing a simple insert where as in second SP you are inserting and then doing an update.
So it is pretty obvious that the second procedure is doing more work