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In a database, one may create a many-to-many relationship between tables by creating a third table which maps the two together using foreign keys. What is the relationship between the third table and the two original tables?
For example, if table A and table B have a many-to-many relationship and table AB is the pivot table, is the A->AB relationship always a many-to-one relationship, and is the B->AB relationship also always a many-to-one relationship?
I believe the short answer is “Yes”. 🙂
Simply put, foreign key relationships must map an AB column to a single column in A or a single column in B. But since the contents of AB represent the many-to-many relationship between A and B, there can by definition be multiple instances of either the A key or the B key in AB. At the same time, a duplicate record in AB, ie. representing the same A-B relationship multiple times, would be in error.
So, yes: one-to-many on either side of AB.