How to Update a numeric field with the SUM of the numeric field in a different table MS-ACCESS 13

Question

I have a supply inventory database with 3 tables. Main (Warehouse Stock), Supply Issue Table, and Supply Purchases Table. All three tables share the relationship ITEM #. Supply Issue and Supply Purchases both log transactions (ie Item #, Quantity Issued or purchased). I need to have the Purchase and Issue tables update the total quantity in the Main Table

I created an update query to update the two fields in the main table(PQuantity/IQuantity). Then a third auto-calculated field (MQuantity) which is (PQuantity-IQuantity). I did it this way because I understood I couldn't run an update query for an auto-calculated field?

Here's the update query SQL for purchases:

UPDATE [MAIN (WAREHOUSE STOCK)] 
INNER JOIN [Supply Purchases Table] 
ON [MAIN (WAREHOUSE STOCK)].[ITEM #] = [Supply Purchases Table].[ITEM #] 
SET [MAIN (WAREHOUSE STOCK)].PQUANTITY = +[Supply Purchases Table]![QTY  ADDED];

Here's the update query SQL for issues:

UPDATE [MAIN (WAREHOUSE STOCK)] 
INNER JOIN [Supply Issue Table] 
ON [MAIN (WAREHOUSE STOCK)].[ITEM #] = [Supply Issue Table].[ITEM #] 
SET [MAIN (WAREHOUSE STOCK)].IQUANTITY = +[Supply Issue Table]![QTY ISSUED];

The problem with this is it will only total the last purchase or issue that was input. I also get "Type conversion" errors.

Am I making this too difficult?

Thanks!

Answer 1

You have to add QTY ADDED to the old value of PQUANTITY like:

UPDATE
    [MAIN (WAREHOUSE STOCK)]
    INNER JOIN
        [Supply Purchases TABLE]
    ON
        [MAIN (WAREHOUSE STOCK)].[ITEM #]  = [Supply Purchases TABLE].[ITEM #]
    SET [MAIN (WAREHOUSE STOCK)].PQUANTITY = [MAIN (WAREHOUSE STOCK)].PQUANTITY + [Supply Purchases TABLE]![QTY ADDED];

But you should rethink you data structure, as usuallay you have only one table for transactions (IN/OUT) and compute the actual stock when needed. See Ms Featured Access Templates Inventory and Allen Browne AppInventory for some hints.