Re: A (False?) Reason that Many-to-Many Must Have at Least One Descriptive Attribute
Posted by: Bill Karwin
Date: December 12, 2006 12:12PM

That algorithm finds redundant FD's, but it would leave the Book_Lang table with _no_ FD's. {book,lang} -> book and {book,lang} -> lang _both_ qualify as redundant by that algorithm.

Which makes me think, is it a requirement that a many-to-many table have any FD's?

Maybe that's a reasonable definition of a many-to-many table: it has no FD's. This is because the rows of a many-to-many table are not actually entities in the logical data model. A many-to-many table is a physical model construct that we use to implement a logical many-to-many relationship.

Regards,
Bill K.

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