Database Normalization

I'm setting up a new database with the following types of orders

Purchase Orders (sent to manufacturer for inventory)
Sales Orders (received from customers)

The manufacturers will get reports of when/where their items are sold but I don't understand how I'll connect the two for reporting purposes

If I order 5 widgets for inventory at different costs over time - and 3 are sold - how do I know which widget went where....

Currently I have a field "SO_ID" on every "PO_ID" - and a "PO_ID" field on every "SO_ID" so when an item is picked and the serial number is captured they sync-up - but it just doesn't seem like the best way to do this.  It seems like the way I do it now has repeated data and that's what normalization tries to avoid.

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Think you should have a third table for inventories, which will contain both po_id and so_id.  Each widget will have a row on the table and will also act as the join table between purchases and sales.

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ProdigyOne2kAuthor Commented:

Now when trying to re-do our database I'm trying to follow all of the "rules" I'm reading about...and something I read was that no column should be "null".  Maybe this isn't a *must* - but just wanted to check that out as well

Of course the inventory will have a PO_ID right away - but the SO_ID would be "null" until it was committed to an order - is that a concern having a "null" SO_ID?
I don't know where you read that -> it's generally okay for a column to have null values unless it is the primary key.
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