Question

This question has been asked before but never really answered (at least the way I want to do it). I have three tables in my database, threads, responses and votes. I need them to share the id field only. So I can do something like

SELECT * FROM threads AS t JOIN responses AS r JOIN votes AS v
WHERE id = 15

and will only retrieve a thread a response or (exclusive) a vote record with id = 15.

Is there a possible way to do this without creating an extra table? I’m not asking if it’s a good idea (which is probably not), but if it’s possible and how to do it.

Answer 1

Your select will throw an error, because the id field in the where clause is ambiguous. I suppose there is an id field in all of these three tables.

SELECT * FROM threads AS t JOIN responses AS r JOIN votes AS v
WHERE id = 15

So even if the id field is “somehow shared” between these fields, you will have to make your where clause unambiguous:

SELECT * FROM threads AS t JOIN responses AS r JOIN votes AS v
WHERE t.id = 15

Furthermore, I suppose threads and responses have a many-to-one relationship which means there can be several response corresponding to a single thread. This means the responses table would have a thread_id field. The id field in the responses table could not have the same value as the id field in the corresponding thread record, since the id field must be unique in both tables.

The same logic goes to the relationship between responses and votes (a single response would have many votes).

Hence I conclude there is no possible way to share the id field between these three tables.