1. Associative Entities A relationship cannot have attributes
What happens when one or more attributes exist for a relationship?
Employee – Project – Billing Rate

2. Associative Entities
An associative entity is an entity type that associates the instances of more or more entity types and contains attributes that are specific to the relationship between those entity instances.

3. Converting a relationship to an associative entity
Example
Employee’s billing rate can vary by project (employee to project, many-to-many)
Billing
Rate
Employee
Project

4. Example:
Attribute of the relationship between property and owner is percent owned %
Owned
Owner
Property

5. Ternary Relationship
simultaneous relationship among one instance from each of exactly three entity sets
An associative entity is not required but is typical.

6. Ternary Relationships – Cardinality
“one” – one instance of an entity is associated with each unique pair of the other two entities.
“many” – more than one instance of an entity is associated with each unique pair from each of the other two entities.

7. Example (one-to-one-to-one):
Employee is assigned a phone number for a project. A phone number is used only for that employee and project.

8. Example (one-to-one-to-one):
Employee
Has
Project
Phone
Number
Note: An employee could still be assigned to multiple projects but would have a unique phone number for each project assignment.

9. Example (one-to-one-to-many):
Employee assigned to a project works at one location for that project but can work at different locations for different projects. At a location an employee only works on one project, but there can be many employees working on that same project.

10. Example (one-to-one-to-many):
Employee
Has
Project
Location

11. Example (one-to-many-to-many):
Employee on a project has one manager. Manager can manage several projects. Each project has one manager. Manager can manage the same employee on different projects.

12. Example (one-to-many-to-many):
Employee
Has
Project
Manager

13. Example (many-to-many-to-many):
Employees use many skills on many projects and each project has many employees with varying skills.

14. Example (many-to-many-to-many):
Employee
Has
Project
Skill

15. Ternary Relationships
Why not just make the ternary relationship an associative entity?
Location
Employee
Project

16. Good idea if possible. However,
“Business Rules”
Need for separate (strong) entity
What if location also related to other entities?
No longer valid as associative entity
If you can safely get rid of ternary (or n-ary) relationships, DO IT.
However, don’t trade correctness for ease of implementation.

17. Example:
Employee assigned to a project works at one location and is only assigned to one project. However, projects can be at more than one location and have multiple employees working on them at each location. Multiple projects can be at the same location.
Draw the E-R Diagram.

18. Update your E-R Diagram.
Example:
Now, suppose employees have a billing rate that varies with both project and location.
Update your E-R Diagram.

19. Generalization/ Specialization
Generalization: Defining a general entity type from a set of more specialized ones (bottom-up).
Specialization: Defining one or more specialized entities from a more general one based on distinguishing characteristics (top-down).

20. Examples:
Generalization: Bank account generalized from checking, savings, and loan.
Specialization: Property for sale divided into single family, duplex, apartment, commercial, or industrial.

21. Why use generalization or specialization?
clarity
more fully describe situation
conversion to OO (inheritance)
different relationships only apply to specific subtypes
monthly charges only made to checking accounts
different relationships only apply to the supertype
customer must have an account with the bank
different (additional) attributes for the subtypes

22. Generalization/ Specialization
Applies to entities only
Types:
disjoint – can be only one subtype
overlapping – can be more than one subtype
Completeness Constraint – subtypes fully inclusive of supertype

23. Generalization/ Specialization
Disjoint – student can be undergraduate, masters, or doctoral
Overlapping – major can be history, philosophy, or mathematics

24. Example
A company sells products whose price can change at most once a day. Need to be able to track the price history.

25. Multivalued Attribute
Product ID
Product
Description
Price
Price History
Effective Date
Product Type