This type of setup is semi-common and I've seen it called "polymorphic relationships". The PHP framework Laravel has first-party support for them if you want a

This type of setup is semi-common and I've seen it called "polymorphic relationships". The PHP framework Laravel has first-party support for them if you want a real example, although those are generally only polymorphic for one side of the relationship (i.e. building a CRM and having a single "comments" table with the ability to relate Customers, Opportunity, Leads, etc. to each comment without having a unique column for each type). Generally, the relationship table will look like this: - relation1_id - relation1_type - relation2_id - relation2_type Where *_id is the database ID and *_type is some kind of string constant representing another table. Generally you put an index on "relation#_id, relation#_type", but that's obviously use case dependent