For now ignore in this sample code the column 'rec_id'

declare @Used decimal (12,2)
set @Used = 7
create table #invstock (inv_code char(20), qty_left decimal(10,2), inven_pk int, unit_price decimal (12,2), rec_id char(1))
go
insert into #invstock (inv_code, qty_left, inven_pk, unit_price, rec_id) values 
	('ABC', 15, 1, 1.93, '1')
insert into #invstock (inv_code, qty_left, inven_pk, unit_price, rec_id) values 
	('ABC', 4, 2, 1.93, '1')
insert into #invstock (inv_code, qty_left, inven_pk, unit_price, rec_id) values 
	('ABC', 7, 3, 1.93, '1')
insert into #invstock (inv_code, qty_left, inven_pk, unit_price, rec_id) values 
	('ABC', 8, 4, 1.93, '1')

I need to create a SQL Update that will change the value of each record column QTY_LEFT subtracting the value of @Used.
That is, if the value of @Used is 7, only the first row will change the value of QTY_LEFT to 8. But if the value of @Used in 20, then the first row will have the value of QTY_LEFT as 0, the second row will be 0, and the 3rd one will have QTY_LEFT as 6.

This is one small part of the overall SQL I am trying to create but I need to break it down so that I can understand it.

>Can you provide more details?
>