Principles of Database 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 n An entity is represented by a rectangle containing the entity’s name. An entity corresponds to a table in the relational environment. n A relationship is an association between entities. It is represented by a line connecting the two entities. n Cardinality expresses the specific number of entity occurrences associated with the related entity.

Relationships Types of Relationships n One-to-One e.g., one dean to one university one head-of-state to one country n One-to-Many e.g., one painter paints many paintings one customer places many orders n Many-to-Many e.g., students enroll in courses

ER Diagrams with Crow’s Foot Notation and Cardinalities

ER Diagram Problem n Acme Insurance Company is made up of several divisions. A claims adjuster is always assigned to one and only one division. n 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. n 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 Edition 7 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 n For many-to-many relationships, you must create a separate intersection table to handle the relationship. n Access will not let you define a many-to-many relationship directly between two tables. n The primary key of the intersection table is a composite primary key. n Two one-to-many relationships are created.