Re: BCNF: superkey or candidate key ?
From: Jan Hidders <hidders_at_gmail.com>
Date: 27 Sep 2006 01:59:23 -0700
Message-ID: <1159347563.245658.14870_at_k70g2000cwa.googlegroups.com>
> Here's what's behind my question. relational tables with at most 1 non-key
> column is where the great debate about NULLs becomes moot. If you want to
> leave the non-key column NULL, then just omit the row. Of course, you need
> another table with just the primary key to keep track of which of the
> possible primary keys are in existence.
Date: 27 Sep 2006 01:59:23 -0700
Message-ID: <1159347563.245658.14870_at_k70g2000cwa.googlegroups.com>
David Cressey wrote:
>
> Here's what's behind my question. relational tables with at most 1 non-key
> column is where the great debate about NULLs becomes moot. If you want to
> leave the non-key column NULL, then just omit the row. Of course, you need
> another table with just the primary key to keep track of which of the
> possible primary keys are in existence.
Why the restriction to at most 1 non-key column? It doesn't matter how many non-key columns there are: you can always apply that transformation if the nullable column is not part of a candidate key.
One might even argue that in theory the restriction to non-key columns is not really necessary. In that case you just need to introduce a more complex database constraint to replace the candidate key constraint.
- Jan Hidders