Monday, September 3, 2012

Conceptual Business Modeling for Database Design: Entity Supertype-Subtypes

See the update on 1/5/16.

Meaning Criteria and Entity Supertype-Subtypes 






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2 comments:

  1. Interesting post! I can tell you that after building a system for an auction mgmt system that did NOT use property-sharing entities, I learned my lesson and ended up building a second system for a martial arts concern that did. The auction management system has separate bidder and consignor tables and in real life some bidders ended become consignors. By not using property-sharing entities I ended up having to duplicate records across both tables. It wasn't the end of the world, but clearly could have been thought out better. My second major build used a contact table, just as you mentioned, along with student, instructor and billee tables that each had an fk to contact. This has worked well and has reduced duplication. I think when designing your system the key question to consider is the role of these contacts in your system and how they might evolve over time. If a contact will only ever exist as one "type" forever (i.e. a student will only ever be a student, faculty will only ever be faculty) then having separate tables make sense. But if that contact (and contact is a much better name for the table that "entity") could ever exist as more than one "type" then it certainly makes sense to use property-sharing entities .

    As for the need for logging? Yes, you can go through the trouble, but I've never had a client tell me that they needed to know what address a contact was using 4 years ago. As long as the information for the contact is current that's what seems to matter most.

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    Replies
    1. Database design is driven by the conceptual model. How good the latter is determines how useful the former is.

      Practitioners tend to conflate the conceptual with the logical -- they rush into design with poorly specified models.

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