I have the table Items which is a child of Platform. Each platform can have multiple items. I need to make a new table, which I will call WPE which can also have multiple items. What would be the best way to make this work. Should I make a new foreign key in Items?

Thanks

if you want Items to be a child of WPE, then the answer is yes

Thanks. So it is ok for items to be a child of platform and WPE? I was wondering if this is a situation where you needed to use a junction table.

sure, in exactly the same way that it's okay for a product to be a child of a vendor at the same time as being a child of a product category

probably not

but note that if both of the FKs are declared as NOT NULL, then the items table is a junction table between platforms and WPEs

Some items will not be a child of WPE, so it can be null. So items will not be a junction table. My items table looks like it would be a junction table.

I would do it like this and without nulls:

WPE {WPEId} KEY {WPEId}
Item {ItemId, PlatformId} KEY {ItemId} (or {ItemId, PlatformId}??)
ItemWPE {ItemId, WPEId} KEY {ItemId} (or {ItemId, WPEId}??)

You don't need nulls to represent any kind of relationship.

It would look something like this:

Platform - WPE
| |
Item-------

But each item may not be a child of WPE at all.

That didn't come out correctly the second pipe is suppose to come down from WPE

I think my suggestion is pretty close. You just need to decide whether the relationships are 1-many or many-many and choose your keys accordingly.

If that particular item has no WPE then that foreign key is null. Correct?

If an item has no WPE then it won't have a row in the ItemWPE table at all. No need for a row with a null in it.

I see what you are doing. I thought of something similar. I just needed some direction on which route to take. Thanks!