MySQL：是否可以对两个不同的表列创建外键？

Question

I have a table (a) which should have a column "user_id", which is a foreign key of some other table (b) OR (c). So wether it points to table b or c (only one, not both).

Is this even possible in MySQL? Best thing I can come up with is following (but as you can see, I have "user_id" twice in my "a"-table, which is not very nice, since one of the two values will always be empty.

Thank you very much!

Answer 1

Hi this is called Polymorphic Associations and you cannot enforce it using MySql foreign key constraints.

in order to have either userId_a null or user_id_b null you can create a check constraint

CONSTRAINT CHECK (user_id_a IS NOT NULL OR user_id_b IS NOT NULL)