I'm looking for the"best" way to maintain my inventory count. I am developing a POS system for my retail store.

I have an inventory db as well as a line item db for individual sales.

What is the best approach to update the quantity in the inventory db from the line item quantity sold?

Thanks!

The best way is not to have an inventory total count as such. High end systems never debit inventory directly. They just enter a transaction in a file for each draw or stocking operation. This way you can always reconstruct inventory quantity, the worst that can happen is that an incorrect transaction is posted. If you store a single quantity number and change it incorrectly, there is no way to recover from the error, except to review all transactions elsewhere and do a physical count.

-bd

I.e., relate your Products file to your Line Items and your Invoices to your Line Items.

Then you can add stock of a given item via the Products file, via a portal or a script that creates a new record in Line Items. You subtract stock the same way, but via the Invoices file. Each Line Item record has a positive or negative quantity, so you just add them up, either by summary fields in a Line Items report, or by your relationship from Products Sum(Line Items::Qty).