Developing Usable Experiences

Entity-Relationship Variability

October 13th, 2009 7:17 am —  426 views

There comes a time1 every semester where I feel like I’ve entered the Thunderdome. I hear cheering and shouting, the smell of swine is strong, Tina Turner is announcing. My opponent is myself, and I can look pretty mean sometimes.

I’ve gone up against myself like this many times before; I know what kind of fight it means. Drawn out and ugly. And sure enough, I’m drawn out and ugly.

Since I usually have to work things out by talking about them, I’m going to try and articulate this homework problem that followed me into sleep and back. It has to do with entity relationship data modeling.

You’re shown a traffic citation correction notice2 and told there are 5 entities represented by it. You’re asked to break the traffic citation correction notice into “things” that can be “identified” then explain the nature of relationships between them3. How will the numbers of one thing compare to the number of a related thing? Does one thing have to exist for the other, related thing to exist?

Really? Just 5 entities? Is one of them the thing itself, the traffic citation correction notice? I’m having a hard time accepting that there are only 5.

If I were designing a data model to be used for law enforcement purposes across many states, I’d be more particular about how it was engineered. The relationships that appear between things could help solve crimes, save lives, and provide court systems with history and facts.

I’ve been pondering this for a couple days now. While I was writing this I did a fresh sketch of a data model that places no limitations on the number of entities involved.

er-diagram-entitiesWhat has me drawing it out is the fact that the more I think about it and reason through variations of the relationships between things, I find more variations. There are arguments to support good and bad design decisions and my mind gets stuck in this infinite loop of puzzling.

And my dreams are a drawn out puzzle where I’m shifting, turning and twisting pieces, looking for that fit.

But it’s due on Thursday. Should I settle on something with 5 entities? It doesn’t feel right. And I have to go to work now.

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  1. …sometimes many times [↩]
  2. Seen above [↩]
  3. And I’m hoping we’re only talking about binary relationships. [↩]

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4 Comments 

Comments

4 Responses to “Entity-Relationship Variability”

  1. DanM on October 13th, 2009 8:24 am

    Focus on the /required/ relationships; either required by the citation for it to exist, or requiring the citation in order to exist (the latter being unlikely, if you’re just looking at the citation). Other potential relationships are the sort of thing that you exploit in ad hoc queries; they’re of no relevance to the logical schema design. (They may later become relevant to physical design for performance reasons.)

  2. DanM on October 13th, 2009 8:27 am

    Umm. Looked more closely at your picture. In an ER model, treat things like the car make and model as attributes of the vehicle entity; they’re unlikely to be significant enough to the domain to treat as separate entities. Consider: Do you think that the judicial branch keeps a definitive list of all car makes and models?

  3. Spike on October 13th, 2009 12:55 pm

    Good points Dan. Thanks for the input.

  4. Dawn on October 13th, 2009 9:26 pm

    You just have to pass the course and finish all this foolishness! OK. Never mind, I know you, and you want to do the best. So follow those guys advice up above. I have NO IDEA what you’re talking about! :)

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