12.1 Constraints on Specialization/Generalization

There are two main types of constraints that define how entities can belong to subclasses.

1. Disjointness Constraint

• Disjoint (d): An entity can belong to at most one subclass.
• Example: An employee can be either "Hourly" or "Salaried", not both.
• Overlapping (o): An entity can belong to multiple subclasses simultaneously.
• Example: A person in a university can be both a "Student" and an "Employee".

2. Completeness (Totalness) Constraint

• Total Specialization: Every entity in the superclass must belong to at least one subclass.
• Notation: Double line connecting superclass to circle.
• Partial Specialization: Some entities in the superclass may not belong to any subclass.
• Notation: Single line.

12.2 Combining Constraints

Possible combinations for a specialization:

1. Disjoint, Total: Must be exactly one subclass.
2. Disjoint, Partial: Can be zero or one subclass.
3. Overlapping, Total: Must be at least one subclass (can be many).
4. Overlapping, Partial: Can be zero, one, or many subclasses.

12.3 Categorization (UNION Types)

A Category (or Union Type) is a subclass that represents a collection of entities from different entity types.

• Example: In a banking database, an "Owner" of an account could be a PERSON, a BANK, or a COMPANY.
• We create a category OWNER which is a subclass of the Union of {PERSON, BANK, COMPANY}.
• An entity in the category inherits attributes from only one of the superclasses it belongs to.

12.4 Conclusion of Unit 1

We have covered the fundamental concepts of DBMS, the architecture that ensures data independence, and the ER/EER models used to design robust databases. These conceptual tools are the foundation upon which all physical databases are built.