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Conceptual/Logical/Physical Modeling

Former Member
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Hi all,

So far, I have been using Logical and Physical modeling only.  I am doing this because we have Oracle/Sybase/SQLServer and Netezza as database backends for our datastores.  The Logical will, as you know, be database agnostic.  This is how I've seen it with Erwin but I'm migrating to using PD and then hopefully my entire team will follow to PD after I've used it successfully on multiple projects.  But, as I watch tutorial videos as well as steal VB scripts from here all of them seem to suggest CDM to PDM and because LDM's aren't even mentioned, so I'm assuming that is a tacit recommendation to sort of bypass using LDMs.  The videos are either Conceptual or Physical with no mention of Logical and the scripts start out asking if this is a CDM.  Coming from Erwin, I know Logical/Physical while I've not dealt with the concept of Conceptual.  And, coming from Erwin, I've never used Logical modeling successfully anyways.  There are many modelers on our team, and we all have to agree with is in logical vs physical and even trying to do it singlely, I've not seen the benefit so we end up all just doing physical.  But, I'm the one driving PD, and I'm pretty confident we will at least be able to use Logical.  I do see the benefit of the model being database agnostic, and I'm sure PD will offer all sorts of benefits there.  But, I'm wondering should I model Conceptual and then generate straight to Physical and bypass Logical? 

Recommendations?

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marc_ledier
Active Participant
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Hi,

Here is the PowerDesigner view of CDM/LDM/PDM:

Usually, one use the conceptual data model (CDM) to define high level concepts of your information modeling.

In CDM, you can define business rules, entities, inheritance, data items and n-n relationships. You don't see the the migrated foreign key -- it is physical/logical implementation -- and nothing specific to DBMS (no view, no index, no storage info).

It is a conceptual view of your whole set of models.

The logical data model (LDM) is middle-level view of your information design.

You still have entities, relationships and inheritances. You can allow n-n relationships (optional).

You will see migrated attribute (FK). what you define is LDM can be seen as a DBMS-independent physical data model with additional conceptual items.

The physical data model (PDM) is the Database view of your information modeling. It contains all the physical objects that the database can manage.

The LDM/PDM couple is the easiest way to start with PowerDesigner for ERwin users.

The CDM/PDM couple is used by long-time PowerDesigner users.

Starting a new project, I would recommend to use the triple CDM/LDM/PDM drilling from conception (from mind) down to database (physical implementation).

You also can divide each level in sub-level using intra-model generation (CDM->CDM or LDM->LDM).

For instance, you can have a precise (and abusive) information definition with following model map

* Define Data items (in relation with a Glossary) in a Top-Level CDM

* Generate a Middle-Level CDM in which you add entities (using data-items as attributes) and relationships.

* Generate a LDM to organize your entities in a pre-physical view

* Generate as many PDM as you have target databases and play denormalization in them as desired.

You can also browse modeling web sites to get usage examples.

Marc

Former Member
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Thanks.  I'm using LDM/PDM for now.  That's enough to keep me busy learning how to get PD to start being Powerful.