Running subtraction in Access Db

Hi, all,

I have an inventory database I'm working on, and want to allocate inventory based on ship date.  For example

Part             Date           Qty On Order             Qty On Hand            Adjusted On Hand
ABC           10/30/08               5                                6                              1
DEF           10/30/08               1                                4                              3
ABC           11/5/08                 3                                1                             -2
DEF           11/6/08                 4                                3                             -1

Where the Qty On Hand for oldest shipments is pulling from an adjusted on hand.  I may have been looking at this too long, and staring an easy answer in the face - but right now I'm stumped...

I appreciate (in advance) any help!

Shauna
orcas828Asked:
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miqrogrooveCommented:
Hi orcas828, what is the question exactly?  Do you need help designing a new table?
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orcas828Author Commented:
Hi, migrogroove,

Sorry if my question wasn't clear.

I have two linked tables to csv files:  BalOnHand and Orders.  I want to be able to allocate inventory to the orders (in order of age), and reduce the balance of the item for newer orders down the list.  

So, if order #1 needs to ship 10/30, with part ABC, qty 5, and balance on hand as 6, a new column would show an adjusted balance of 1 for that part.  Then, for order #5 that needs to ship 11/5, for part ABC, qty 4, I'd show a -3 in the adjusted balance.

I'm not sure what I'd need to do to make that happen.

Hopefully, I've explained my question a little better!
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orcas828Author Commented:
Kinda like a reverse DSUM (only subtracting for each part, instead of adding...)
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miqrogrooveCommented:
Okay so you just need a running sum.  When you say "a new column would show an adjusted balance of 1 for that part" does that mean it must be calculated and stored in a table?  Or do you only need the running sum to appear in a report?
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orcas828Author Commented:
At this point, I only need it to show in a report.
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miqrogrooveCommented:
Excellent.  That will give you a lot of flexibility and a simple answer:

Create a report that has the Qty field listed twice.  One of the two instances will become the running sum or balance column.  In the properties for that field, look at the Data tab and you will see a Running Sum property.  Set this property to "Over All" or as appropriate.  This will give you a running sum of that column in the report output.

Depending on the circumstances, you might need a starting quantity or "balance forward" row in the underlying data.  This can be accomplished with a relatively simple union query.
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orcas828Author Commented:
Will the running sum do a running subtract, though?
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miqrogrooveCommented:
Yes I think one of the nuances here is that you want to display a positive quantity ordered but have a negative running sum.  So for simplicity this report should be based on a query that has the qty ordered column also listed twice.  In the Field row of the query's design view for one of the two columns, enter something like:

QtyChange: -[QtyOrdered]

Now if you set the data source for the 'running sum' field to QtyChange instead of QtyOrdered you will get a negative running sum.

If you are adding to your inventory by receiving shipments then you will want to consider where and how that gets represented.  Ideally you want to end up with a QtyChange column that can make an elegant running sum, even if it is necessary to do a union between orders and receiving.
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