What is the Extended Entity-Relationship Model (EERM)?
The extended entity-relationship model (EERM) is the result of adding more semantic constructs to the original entity-relationship (ER) model.
Entity Supertype and Entity Subtypes
Now let’s assume it’s a factory. There are many different kinds of workers working in this factory. For example, technician, engineer, accountant, manager. Although they are all workers, the areas they are interested in are different. So we are grouping our working-class into sub-branches. Here we have entity supertype and entity subtype.
An entity supertype is a generic entity type that is related to one or more entity subtypes, where the entity supertype contains the common characteristics, and the entity subtypes contain the unique characteristics of each entity subtype.
EMPLOYEE -> Entity Supertype ACCOUNTANT,ENGINEER,TECHNICIAN,MANAGER->Entity Subtype
Entity supertypes and subtypes are organized in a specialization hierarchy, which depicts the arrangement of higher-level entity supertypes and lower-level entity subtypes.
A specialization hierarchy provides the means to:
- Support attribute inheritance.
- Define a special supertype attribute known as the subtype discriminator.
- Define disjoint/overlapping constraints and complete/partial constraints
What is a subtype discriminator?
A subtype discriminator is an attribute in the supertype entity that determines to which subtype the supertype occurrence is related.
Disjoint and Overlapping Constraints
Disjoint subtypes ( non-overlapping subtypes ) are subtypes that contain a unique subset of the supertype entity set. If we look at the example table at the top, an employee who is a pilot cannot also be an accountant. This is an example of a Disjoint constraint.
Overlapping subtypes are subtypes that contain nonunique subsets of the supertype entity set. You can also do your master’s degree while working at a firm. In this way, you become both workers and students.
Constraints are set by business rules.
Partial and Total Completeness
Partial completeness (symbolized by a circle over a single line) means that not every supertype occurrence is a member of a subtype; that is, there may be some supertype occurrences that are not members of any subtype.
Total completeness (symbolized by a circle over a double line) means that every supertype occurrence must be a member of at least one subtype.
Resources
http://www.myreadingroom.co.in/notes-and-studymaterial/65-dbms/492-subtype-discriminator.html
Database Systems: Design, Implementation, and Management Eighth Edition Chapter 6
