create table tbl_invoice
(id int identity(1,1)
,invoice_id int
,amount money
);

create table tbl_invoice_log
(logID int identity(1,1)
,LogDate smalldatetime
,log_action char(20)
,invoice_id int
,current_amount money
,new_amount money
,change_amount money
);



insert into tbl_invoice
select 51341, 431.53 union all
select 51342, 1251.00 union all
select 51345, 52
--(3 row(s) affected)


--only update trigger
create trigger trg_tblInvoice_update
on tbl_invoice
after update
as
insert into tbl_invoice_log (LogDate, Log_action, invoice_id, current_amount, new_amount, change_amount)
select 
	 getdate() as LogDate
	,'UPDATE' as Log_action
	,d.invoice_id as Invoice_Id
	,d.amount as current_amount
	,i.amount as new_amount
	,i.amount-d.amount as change_amount

from inserted as i, deleted as d
where
	 i.invoice_id = d.invoice_id

	

--intial data
select * from tbl_invoice
select * from tbl_invoice_log

-- perform test
update tbl_invoice
set amount = 1300
where
invoice_id = 51342


--after change
select * from tbl_invoice
select * from tbl_invoice_log