1. Class Agenda – 04/04/2006 Discuss database modeling issues
IS 475/675 - Database Design
Class Agenda – 04/04/2006
Discuss database modeling issues
Review homework ERD’s
Learn concepts and ERD notation for data generalization
Learn use of Visio for ERDs

2. “Syntax” Database Modeling Issues
Does the diagram contain entities, attributes, primary keys and relationships?
Is it structured using one of the two modeling options discussed in this class? (see the two options on the next two slides)
Does each entity have a primary key?
Is each relationship labeled with a verb or verb phrase?
Does each relationship have a foreign key?

3. Option 1: Chen Notation

4. Option 2: IEW and Visio Notation

5. “Logic” Database Modeling Issues
Is all necessary data represented on the model?
Is the data grouped correctly with the appropriate entity?
Does each entity have a primary key that will yield a unique value for each row?
Do any entities contain data that will repeat and create a variable number of columns for that entity?
Are the entities related accurately?
Are the cardinalities of the relationships accurate?
Are there any M:N relationships?
Are the foreign keys accurate and is each placed in the correct entity to sustain a relationship?
Are there any unnecessary 1:1 relationships?

6. What is generalization?
Generalization is the concept that some entities share attributes.
Example:
Imagine a university human resource system with an entity called “employee”.
An employee has attributes such as name, address, SSN, birth date, educational level, etc.
Different types of entities have other information.
For example, a “faculty” entity has grant type.
A “classified” entity stores data about employment level.
An “administrative” entity stores data about contract type.

8. Generalization vocabulary
An entity “supertype” is an entity whose instances have attributes that are common to one or more entity “subtypes”.
Attributes that are shared by all entities (including the identifier) are associated with the supertype.
An entity “subtype” is an entity whose instances inherit some common attributes from an entity supertype. An entity “subtype” adds other attributes to those inherited from the supertype.
A subgrouping of the entities in an entity type which has attributes that are distinct from those in other subgroupings.
Generalization allows an analyst to understand more fully the relationships between entities.

9. D

10. What are the generalization constraints?
Constraints are limitations depicted on a data model.
The most common types of constraints are:
Completeness. Addresses the question of whether an instance of a supertype must also be a member of at least one subtype.
Result is either required or not required.
Specified on the ERD with the letter “C” under the arrow pointing to the supertype.
Disjointness. Addresses the question of whether an instance of a supertype may simultaneously be a member of two or more subtypes.
Result is specified as disjoint or overlap.
Specified on the ERD with the letter “D” under the arrow pointing to the supertype.