Re: Many to many
Date: Tue, 5 Dec 2000 13:08:49 +0000
Message-ID: <cQO9EyAhjOL6EA6J_at_postar.co.uk>
In article <3a2bffba.52190770_at_news1.telia.com>, Arne Jonsson
<arne.b.jonsson_at_telia.com> writes
>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.
This is true. But when you create the new table you should take a close look at it and see what real-world entity it corresponds to. You may find that there are other fields that should also be in this table.
> 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.
The two fields are always foreign keys. That's their function in the link-table.
You shouldn't ever see a situation where just one of the fields is a primary key in the link-table. If that happened it would mean that there is a 1:1 relationship between one of the parent tables and the link-table.
It's physically possible to implement that in a relational database, but I can't imagine a situation where I might want to. (But I'm sure someone will be along soon with an example.)
>This have made me confused.
I can understand that.
-- Bernard Peek Not necessarily speaking for: POSTer Audience Research -- POSTAR Ltd. 27 Sale Place, London, W2 1YR. (020 7479-9700) Bernard_at_postar.co.ukReceived on Tue Dec 05 2000 - 14:08:49 CET