Entity Relationship Diagrams Objectives s Learn the Elements of the E-R model (entities, attributes, and relationships) s Show how to apply the E-R model for modeling business situations s To develop data modeling skills s To interpret E-R diagrams

Entities s Entity is something that can be identified in the user’s work environment s Usually a noun s Entities are grouped into classes or sets s A particular entity is an instance of the entity class s Entities are usually depicted in an E-R diagram as a rectangle with the Entity name in capital letters

Attributes s Attributes (sometimes called properties) are characteristics that describe entities s Identifiers are attributes that describe an entity instance s Identifiers may be unique or non-unique s Composite identifiers consist of two or more attributes (ex: phone number: area code and number)

Relationships s Relationships are associations between entities s Relationships are sometimes referred to as “Has-a” or “Is-a” s Three types of binary relationships –One to one –One to Many –Many to Many

Cardinality s The maximum and minimum number of entities that can occur on one side of the relationship.

Entity Relationship Diagrams s There are many diagramming conventions--based on CASE tools (several will be illustrated in class) s Entity--Capital letters in a rectangle s Relationships--Diamonds s Maximum Cardinality--inside the diamond s Minimum Cardinality –Hash mark to indicated entity must exist –Oval to indicate entity may or may not be required

Attributes s Attributes can be shown as ellipses or in lists--see figure 3-6

Weak Entities s A weak entity is sometimes called a dependent entity s A weak entity cannot exist without another type of entity exists s A special type of weak entity is an ID- dependent entity s See figure 3-7 for examples

Subtype Entities s Some entities contain option sets of attributes or subtypes s Structures of subtypes and supertypes are called generalized hierarchies s The relationship is called a IS-A relationship s Subtypes inherit from supertypes s The subtypes may be mutually exclusive or not s See figure 3-8

Business Rules s Business Rules may or may not be included in an E-R diagram; depending on whether the application should enforce the business rules or the rules be enforced manually