Re: One-To-One Relationships

From: Gints Plivna <>
Date: Tue, 30 Oct 2007 00:21:28 -0700
Message-ID: <>

On 30 Okt., 07:53, "Phil Reynolds" <> wrote:
> One thing that's not clear to me is when it's appropriate to create a
> one-to-one relationship. I mean, in some cases it's obvious, if there's a
> set of data that wouldn't always apply; then you'd want to create that set
> of fields in a separate table with a one-to-one relationship. But in what
> other cases? After the number of fields in a table is greater than X?
> I'm just curious about what thoughts/theories/ideas people have about
> one-to-one relationships, because that's something that's never been
> entirely clear to me.
> Thank you.

I've done it in two cases:
1) when I have a feeling (or even promise) that today's requirements might change in the future and the 1:1 cardinality might be 1:n after a year and
2) when entity (and later table) contains different kind of attributes and application/user usually will be interested in only part of them. So probably I'd separate all attributes in two logical parts, two entities with realationsip 1:1.

However such cases are quite rare.

I've never done that only because there are too many attributes in an entity.

Gints Plivna Received on Tue Oct 30 2007 - 08:21:28 CET

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