Dmitry,

With all due respect, you're taking the wrong approach. Yes, you can do this with a do while or even with (ugh) records based solution.
Best is to calculate on the fly as I showed you earlier.

By having seperate records for purchases and seperate records for sales, you keep way better track of what is happening and is faster than what you're trying to do now and you can calculate the stock in a single SQL statement of 10 lines.



>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?
>>