rahul.katke

 Profile Answers by rahul.katke 

• Dec 27th, 2007

 

The Logical Data Model (LDM) is derived from the Conceptual Data Model (CDM).

The CDM consists of the major entity sets and the relationship sets, and does not state anything about the attributes of the entity sets.

The LDM consists of the Entity Sets, their attributes, the Relationship sets, the cardinality, type of replationship, etc.

The Physical Data Model (PDM) consists of the Entity Sets (Tables), their attributes (columns of tables), the Relationship sets (whose attributes are also mapped to columns of tables), along with the Datatype of the columns, the various integrity constraints, etc.

Erwin calls the conversion / transformation of LDM => PDM as Forward Engineering which further leads to the actual code generation and the conversion of Code => PDM => LDM as Reverse Engineering!

Though the question says specifically about Erwin, the above explanation is independent of the datamodeling tool used for the purpose. 

SashaFroyland

 Profile Answers by SashaFroyland 

• Mar 1st, 2008

 

The only difference is in what's been selected in the model type when the
diagram was first started.

In ERWin 7.3 you have the choice of creating three different kinds of
diagrams:
1) Physical,
2) Logical;
3) Physical/Logical.

In order to have a Logical diagram with entities which automatically transfer
to the physical diagram then you must start with a Model that is of the type
Physical/Logical.

After you create entities in the Logical model, you then click the down arrow
on the top middle of the ERWin GUI to select Physical, and ERWin changes to the
physical model and the logical entities you created in the local model are
automatically in the physical model - unless you tagged the logical model
entities to only be in the logical model.

Another difference may be that in the logical model a many-to-many
relationship may not contain the node table but in the physical model the
many-to-many relationship should contain the node table.