1. MBA 664 Database Management
Dave Salisbury
( )
(web site)

2. Evolution of the E-R Model
Basic E-R Model
nearly 25 years old
complex data relationships and new database technology have outgrown it in some respects
Enhanced E-R Model
a response to the shortcomings of the basic E-R model
not universally agreed upon in some respects
introduced the supertype/subtype relationship

3. Supertype/Subtype Relationships
Supertype (example: Employee)
a generic entity that has a relationship with one or more subtypes
Subtype (example: Manager)
a subgrouping of a supertype entity that is meaningful to an organization
shares all attributes of its supertype, but also has unique attributes of its own and/or :
has relationships with other entities distinct from those of other subtypes

4. Supertype/Subtype Notation
Delta & Pine Land Company
11/25/97
Supertype/Subtype Notation

5. The student example
STUDENT
UNDERGRAD
STUDENT
GRAD
STUDENT

6. Two Rules for When to Use Supertype/Subtypes
Use this type of relationship when either (or both) of the following are present:
When there are attributes that apply to some (but not all) of the instances of an entity type
When the instances of a subtype participate in a relationship unique to that subtype

7. Attribute Inheritance
“The property by which subtype entities inherit values of all attributes of the supertype.
This important property makes it unnecessary to include supertype attributes redundantly with the subtypes.”

8. Supertype/Subtype Example 1
Hourly_Rate
EMPLOYEE
SSN
Name
City/State/Zip
Telephone
CONSULTANT
SALARIED
HOURLY
Annual_Salary
Stock_Option
Contract_Number
Billing_Rate

9. Supertype/Subtype Relationships in a Hospital

10. Two Processes to Develop Supertype/Subtypes
Generalization
The process of defining a more general entity type from a set of more specialized entity types
A “bottom-up” approach
Specialization
The process of defining one or more subtypes of a general entity based on distinguishing attri-butes or relationships
A “top-down” approach
Both approaches can be used together

11. Supertype/Subtype Constraints
Completeness
Addresses the question of whether an instance of a supertype must also be a member of at least one subtype
Disjointness
Addresses the question of whether an instance of a supertype may simultaneously be a member of two (or more) subtypes

12. Completeness Constraint: Two Possible Rules
Total Specialization Rule (Double-Line)
Specifies that each entity instance of the supertype must be a member of some subtype in the relationship (Example: all STUDENTS are either UNDERGRADUATE or GRADUATE students)
Partial Specialization Rule (Single-line)
Specifies that an entity instance of the supertype is allowed to not belong to any subtype (Example: FACULTY and STAFF are not the only possible members of the entity EMPLOYEE)

13. Completeness constraint
Total specialization
Partial specialization

14. Disjointed constraint
Can an entity instance simultaneously be a member of two or more subtypes?
Disjoint rule
Overlap rule

15. Subtype discriminators
Attribute of the supertype whose value determines to which subtype an instance belongs

16. What does that look like?

17. Yet another example….

18. Supertype/subtype hierarchy
Arrangement of super- and subtypes where each subtype has only one supertype.

19. Another example...