1. MIS 385/MBA 664 Systems Implementation with DBMS/ Database Management
Dave Salisbury
( )
(web site)

2. Evolution of the E-R Model
Basic E-R Model
over 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. Supertype/Subtype Notation
Delta & Pine Land Company
11/25/97
Supertype/Subtype Notation

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

9. Supertype/Subtype Relationships in a Hospital

10. Supertype/Subtype Relationships in a Hospital

11. 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

12. 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

13. 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)

14. Completeness constraint – total specialization
A patient must be either an outpatient or a resident patient

15. Completeness constraint – partial specialization
A vehicle could be a car, a truck, or neither

16. Disjointed constraint – member of only one subtype at a time
A patient can either be outpatient or resident, but not both

17. Disjointed constraint – member of more than one subtype at a time
A part may be both purchased and manufactured

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

19. Subtype discriminator (disjoint)
A simple attribute with different possible values indicating the subtype

20. Subtype discriminator (disjoint)
A composite attribute with sub-attributes indicating “yes” or “no” to determine whether it is of each subtype

21. Supertype/subtype hierarchy

22. Another example….