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The difference between a logical and a physical data model are hard to
grasp at first, but once you see the difference it seems obvious. A logical
data model describes your model entities and how they relate to each other.
A physical data model describes each entity in detail, including information
about how you would implement the model using a particular (database)
product.
In a logical model describing a person in a family tree, each person node
would have attributes such as name(s), date of birth, place of birth, etc.
The logical diagram would also show some kind of unique attribute or
combination of attributes called a primary key that describes exactly one
entry (a row in SQL) within this entity.
The physical model for the person would contain implementation details.
These details are things like data types, indexes, constraints, etc.
The logical and physical model serve two different, but related purposes.
A logical model is a way to draw your mental roadmap from a problem
specification to an entity-based storage system. The user (problem owner)
must understand and approve the logical model. A physical model is the
roadmap from the logical model to the hardware. The developer (software
owner) must understand and use the physical model.
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