I simply meant a key that is intended to refer to another entity, but as you said its common for RDBMSs to enforce that referential integrity, but you can also

I simply meant a key that is intended to refer to another entity, but as you said its common for RDBMSs to enforce that referential integrity, but you can also have a DB like mongo or dynamo which store an ID , but when you de-refer it (look up the other entity by ID, perhaps by an API call to a peer microservice) the item is no longer there. It gets even weirder with denormalization of some of the data where maybe you can get the title or preview of an item, but going (clicking) into it is broken.