Re: Many to many

On 5 Dec 2000 04:10:34 GMT, hidders_at_REMOVE.THIS.win.tue.nl (Jan Hidders) wrote:

>Arne Jonsson wrote:
>> For about two years I´ve been living happily in the belief that if you
>> have a many to many-relation you split it in a third table between the
>> other two, where you make the primary keys of these two tables to a
>> composite primary key in the third. Now, taking a course in
>> SQL-server, I am taught that the two primary keys can be used as two
>> foreign keys in the new table, or as one primary and one foreign key.
>> This have made me confused. Are all the ways right?
>
>No. You cannot implement a many-to-many relationship with a primary key
>and a foreign key. Suppose you have tables A and B with the primary
>keys #a and #b. If you add #b as a foreign key to A then there will
>always be at most one tuple in B associated with every tuple in B. So
>that is not a many-to-many relationship.
>
>--
> Jan Hidders

Does it make any sense to keep the two keys as foreign keys in the third table, not making them to composite primary keys?

Arne Received on Tue Dec 05 2000 - 06:38:16 CET