Principles ofDatabase Design, Part II

AIMS 2710

R. Nakatsu

Entity-Relationship (ER)Modeling

ER Modeling is the process of creating a diagram of the structure of the database by defining entities and relations among them.

The ER model serves as the basic database blueprint.

The ER model can be used as a communication tool.

ER Model Components

An entity is represented by a rectangle containing the entity’s name. An entity corresponds to a table in the relational environment.

A relationship is an association between entities. It is represented by a line connecting the two entities.

Cardinality expresses the specific number of entity occurrences associated with the related entity.

Relationships

Types of Relationships One-to-Onee.g., one dean to one university

one head-of-state to one country One-to-Manye.g., one painter paints many paintings

one customer places many orders Many-to-Manye.g., students enroll in courses

ER Diagrams with Crow’s Foot Notationand Cardinalities

ER Diagram Problem Acme Insurance Company is made up of several divisions. A

claims adjuster is always assigned to one and only one division. Claims adjusters work on several insurance claims. Each

insurance claim is always worked on by a team of at least two but no more than five claims adjusters.

Each claims adjuster is assigned to his/her own company car (no claims adjuster needs to “share” a car). A claims adjuster may elect not to be assigned to a company car. Furthermore, some of company cars are “unassigned”—Acme likes to keep a few spare cars in inventory just in case.

Draw the ER Diagram for this situation. Note: a claims adjuster is an employee who investigates and

processes insurance claims.

Database Systems, 9th Edition7

Another ERD Notation

Linking Tables:1:Many and 1:1

In a one-to-one and one-to-many link, you link a primary key in one table to a foreign key in another table.

For a one-to-one relationship, the foreign key must be unique (in Access you set the Index value to Yes (No Duplicates).

Referential Integrity: A condition in which a child table’s foreign key must have either a null entry or a matching entry in the related parent table.

Linking Tables: Many-to-Many For many-to-many relationships, you must

create a separate intersection table to handle the relationship.

Access will not let you define a many-to-many relationship directly between two tables.

The primary key of the intersection table is a composite primary key.

Two one-to-many relationships are created.