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  1. #1
    Join Date
    Nov 2009
    Posts
    5

    level of data model

    hi

    there are the cdm, ldm and pdm

    some software allow to generate theses three type of model some other not

    i don't see the utility of cdm

    when i check this web page

    Conceptual, Logical, and Physical Data Models

    if find useless the cdm

    any comment?
    why create a cdm when we use crow's foot, baker, ie... notation

    i found usefull cdm for the merise method... but it's only used in france...

    thanks

  2. #2
    Join Date
    Jun 2003
    Location
    Ohio
    Posts
    12,592
    I agree. Your logical model will include the conceptual model.
    If it's not practically useful, then it's practically useless.

    blindman
    www.chess.com: "sqlblindman"
    www.LobsterShot.blogspot.com

  3. #3
    Join Date
    Nov 2009
    Posts
    5
    does somebody create cdm and ldm?
    why?

  4. #4
    Join Date
    Jun 2003
    Location
    Ohio
    Posts
    12,592
    LDM yes. CDM no.
    If it's not practically useful, then it's practically useless.

    blindman
    www.chess.com: "sqlblindman"
    www.LobsterShot.blogspot.com

  5. #5
    Join Date
    Dec 2007
    Location
    London, UK
    Posts
    741
    All depends on what modelling methods you use. The ORM "Rmap" approach for example derives the final database implementation directly from the conceptual model. So getting the conceptual model correct is the most critical aspect of the design process.
    Object Role Modeling

    ER diagrams are very limited in the range of relationships, constraints and other options that can be properly expressed. Typically an ER diagram doesn't capture enough information to allow for complex data modelling as well as an effective implementation of that model. That's why it's common to take a staged approach, where the model is progresively refined and adjusted, beginning with a Description of what's being modelled and ending with a Design of a database.

    Many software development projects today also use Agile methods. In that case I find it useful to create smaller, subject-area models that are shared with the team in each sprint / iteration and that will probably be much closer to what is actually implemented rather than what is conceptually being represented - i.e. model as Design rather than Description.

  6. #6
    Join Date
    Sep 2002
    Location
    UK
    Posts
    5,171
    Without it being a formal process, I would say that I do tend to start with a CDM, i.e. an ERD without the attributes filled in, as a way of understanding the broad data requirements. I don't call it a CDM though, if pushed I'd probably have just called it a sketchy, overview LDM.

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