CREATE TRIGGER nullCorr ON test_empl
INSTEAD OF INSERT
AS
  IF (select idfirm from inserted ins) is null
BEGIN
  INSERT INTO test_empl
    SELECT distinct new.id, new.empid, old.idfirm 
      from inserted new
      join test_empl old on new.empid = old.empid 
      and old.idfirm is not null
END
GO

CREATE 
TRIGGER [get_prev_firmID] ON ohot_source.dbo.test_add_last_val
INSTEAD OF INSERT 
AS
SET NOCOUNT ON
BEGIN
	-- if table is empty
	if ( (select count(1) from test_add_last_val) < 1 )
		BEGIN INSERt into test_add_last_val SELECT * FROM inserted END ;
	ELSE 
	-- if table not empty
	BEGIN 
	insert into test_add_last_val (id, empID, firmID) -- add here all other columns needed for insert except "firmID"
	select new.id, new.empID, -- add here all other columns needed for insert except "firmID"
		CASE WHEN (new.firmID is null and old.oldrow = 1) THEN old.firmID 
			ELSE new.firmID END AS firmID
		from inserted new
		LEFT OUTER JOIN (
		select tt.empID, tt.firmID, 1 as oldrow from 
			(select empID, firmID, ROW_NUMBER() OVER(partition by empID order by tupd desc) rown 
				from test_add_last_val) tt where tt.rown = 1
				) old 
		ON new.empID = old.empID
	END
END;
GO