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.
A final similarity worth mentioning between semantic objects and entities is that both have attributes that define their characteristics. For instance, an entity CUSTOMER may have the attributes Fname, Lname, CustNo. Likewise, a semantic object CUSTOMER may have these same attributes. Attributes give us the description of the complete object or entity.
DIFFERENCES.
A semantic object may have one or more of three types of attributes. A simple attribute such as CustNo has only a single, and in this case, unique, value. Group attributes, which are not so obvious in the entity relationship model, are composites of other attributes. For instance, Address may be a group attribute, composed of {Number, Street, City, State, Zip}. These are attributes that combine to create the group attribute Address. Finally, semantic object attributes (or object attributes) establish a relationship between one object and another. These last attributes are similar to Relationships in an E-R model database. This mean that when a user considers attributes within, for example, a DEPARTMENT object, they also must consider attributes relating to MANAGER, COMPANY, and EMPLOYEE objects, if the object attributes are so defined. This seems to go beyond the E-R model of relationships, and yet seems more easily explainable.
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.
