A logical data model describes the data in as much detail as possible, without regard to how they will be physical implemented in the database. Features of a logical data model include:
- Includes all entities and relationships among them.
- All attributes for each entity are specified.
- The primary key for each entity is specified.
- Foreign keys (keys identifying the relationship between different entities) are specified.
- Normalization occurs at this level.
The steps for designing the logical data model are as follows:
- Specify primary keys for all entities.
- Find the relationships between different entities.
- Find all attributes for each entity.
- Resolve many-to-many relationships.
- Normalization.
The figure below is an example of a logical data model.
Logical Data Model
Comparing the logical data model shown above with the conceptual data model diagram, we see the main differences between the two:
- In a logical data model, primary keys are present, whereas in a conceptual data model, no primary key is present.
- In a logical data model, all attributes are specified within an entity. No attributes are specified in a conceptual data model.
- Relationships between entities are specified using primary keys and foreign keys in a logical data model. In a conceptual data model, the relationships are simply stated, not specified, so we simply know that two entities are related, but we do not specify what attributes are used for this relationship.
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