Question

I am creating a relationship between 2 tables:

The relationship I like to form is between the Inventory an InventoryExtended tables.

The primary key for the Inventory table is InvID (Inventory ID).

The reason why I created the InventoryExtended is becauses only 1% of the inventory items in the Inventory table will need additional or extended fields, the rest will not.
Instead of adding these additional fields in the Inventory table where 99% will be blank for 50 additional fields that I need I decided to create an InventoryExtended table and store the 50 fields there.

The relationship between the Inventory an InventoryExtended table will be 1 to 1.

Meaning, for the 1% of the records in the Inventory table , the InvId will be the same as the InvId in the InventoryExtended table.

My question is that should the InvID in the InventoryExtended table be a FK (Foreign Key) or should it be a PK and a FK?

I am thinking it should be a PK and a FK as there the InvID will be unique in the InventoryExtended table.

Thanks in advance.

Answer 1

You are correct.

The InvID should be a PRIMARY KEY and a FOREIGN KEY as it will be unique in the InventoryExtended table.

This type of relationship is indeed 1:1 or (more accurately) 1::0..1, as only some of the rows in the Inventory table will have a related row in InventoryExtended.

Also note that the InventoryExtended (InvID) should not have the IDENTITY property, even if the Inventory (InvID) has it.