Re: 1-1 relationships

From: Angus Monro <>
Date: Fri, 01 Dec 2000 09:12:58 +1100
Message-ID: <>

Thanks - those issues make sense.

Angus Monro.

Jan Hidders wrote:

> Angus Monro wrote:
> > Given that two types of entities have a one-to-one relationship, are
> > there any good reasons for mapping them to two distinct tables with
> > appropriate foreign-key relationships, versus mapping them into a single
> > table? The theoretical side of me wants to keep them in distinct
> > tables, but I can't see the point of the extra space overhead and the
> > time overhead incurred by needing to do joins betweeen them when
> > querying.
> I don't think it is really that simple. The join-problem is largely
> solved by good indexing. Moreover, if you represent the the two entity-types
> in one table you have to access this big table, even if you only need
> the data from one entity type. So a simple query like SELECT A from R;
> might need twice as much disk access as in the split case.
> Furthermore, the fact that you identified two entity types (and not
> one) in your conceptual data model means that the one-to-one
> relationship may easily one day change into a one-to-many relationship.
> The splitting of tables afterwards will then involve quite some
> rewriting of code.
> --
> Jan Hidders
  Received on Thu Nov 30 2000 - 23:12:58 CET

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