Semantic Model vs. Entity Relationship Model

Both the semantic model and the entity relationship model are methods of documenting and analyzing users’ requirements, and converting those requirements into a database the users can navigate easily through, one which provides forms, reports and queries so that users can track entities important to their work. Once the need for a database system has been decided, the developer determines, along with the users, the “things” to be represented by the database. The developer also specifies characteristics of those “things”, and establishes relationships among them.

In the semantic model of design, those “things” are called semantic objects. Semantic objects are similar to entities in the entit

Semantic objects are grouped together into classes, similar to the way entities are grouped. Semantic object classes, like entity classes, are represented in uppercase letters. Object classes contain instances of objects, just as entity classes contain instances of the entity. Thus, the CUSTOMER object mentioned above would contain instances of customers, for example CUSTOMER JOHN DOE, or more likely, CUST 73852 or the like. We say this because there may be more than one JOHN DOE, but 73852 most likely represents a unique instance, since we want object instances to be unique and identifiable in their own right.

The major difference between the semantic object model and the E-R model, however, is the way objects are represented in diagrams. On the surface this may seem shallow, but the diagrams each model uses goes to the very essence of how these models are used. The following illustration is an entity relationship model diagram of a terrorist tracking database:

The semantic object model is very similar to the entity-relationship model. Both look at business needs, and translate those needs into “things” that need to be tracked. However, the real difference lies in how the models look on paper. Compare the semantic object model to the E-R model, with its mostly text based representations, and its individual relationship diagrams, and the semantic model begins to look much more elegant and informational. The semantic object model provides us with a compact, logical way of looking at our data and laying it out. While the E-R model gives us much info

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