Postgres: How Can I Make a JOIN/Junction Table Which References *One Of* Two Other Tables?

Question

I have a Postgres database, and it has a Foo table and a Foo_Group table. It also has a Bar table, and all of these tables have a simple id primary key column.

I'd like to create a many-to-many relationship between the three, such that a Bar can have 0+ Foo s and 0+ Foo_Group s. I can create a Foo_and_Foo_Groups_Bar JOIN table easily enough, with foreign keys to Foo , Foo_Group , and Bar . However, I'm having trouble creating the primary key on my JOIN table.

The problem is, in Foo_and_Foo_Groups_Bar either foo_id or foo_group_id will always be NULL ... but primary key columns can't have NULL s. The only solution I've been able to find is to put an id column on Foo_and_Foo_Groups_Bar ... but I thought JOIN tables weren't supposed to have their own ID, and were supposed to use their JOIN ID columns as the primary key.

So what is "correct" here? Do I add a (seemingly unnecessary, except to make the constraint happy) column to my JOIN table, or is there a better way?

Answer 1

I am a fan of synthetic primary keys. You can add a serial / generated always as identity column for a primary key. And then add a unique constraint for the triplet of ids.

I'm not 100% sure from your example that you are talking about a single relationship. It sounds like you have two relationships, one between bar and foo and another between bar and foo_groups . If that is the case, then you can have two bridge tables and don't have to worry about NULL values.