Re: 1-1 relationships

>>>>> "Angus" == Angus Monro <ajmonro_at_ingennia.com.au> 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				                  av_at_cs.uta.fi
University of Tampere, Department of Computer and Information Sciences
Tampere, Finland				      http://www.cs.uta.fi/~av/