Re: 1-1 relationships

From: Arto Viitanen <>
Date: 30 Nov 2000 08:59:27 +0200
Message-ID: <>

>>>>> "Angus" == Angus Monro <> writes:

 Angus> Given that two types of entities have a one-to-one relationship, are
 Angus> there any good reasons for mapping them to two distinct tables with
 Angus> appropriate foreign-key relationships, versus mapping them into a
 Angus> single table?  The theoretical side of me wants to keep them in
 Angus> distinct tables, but I can't see the point of the extra space
 Angus> overhead and the time overhead incurred by needing to do joins
 Angus> betweeen them when querying.

Check Batini, Ceri, Navathe: "Conceptual Database Design, An Entity-Relationship Approach", Benjamin/Cummings 1992, chapter 12.2.4. They list two cases when it is suitable to make them a one table and two cases when it is more suitable to make them separate tables. The main reason for one table is the full participation: if both entitities are always in pair, why not put them together?

Arto V. Viitanen				        
University of Tampere, Department of Computer and Information Sciences
Tampere, Finland
Received on Thu Nov 30 2000 - 07:59:27 CET

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