Chapter Five Data Modeling with the Entity-Relationship Model

The Data Model A data model is a plan, or blueprint, for a database design. It is more generalized & abstract than a database design. It is easier to change a conceptual data model than it is to change an implemented database design. Modeling is not easy You must abstract the important details of an organization.

Entity-Relationship Model The ER model consists of a set of graphical symbols that are used to create conceptual models of databases. We’ll start with the very straightforward Chen model and then move to Erwin, preferred by our textbook

Entities Things that can be identified and that users want to track Entity class — a collection of entities of a given type Entity instance — the occurrence of a particular entity There are usually many instances of an entity in an entity class. Entities translate into database tables.

Attributes Attributes are an entity’s characteristics. All entity instances of a given entity class have the same attributes, but vary in the values of those attributes. Attributes may be Multivalued Derived Identifiers

EMPLOYEE Entity with Attributes

Identifiers Identifiers are attributes that uniquely identify entity instances. May be simple or composite. Identifiers in data models become keys in database implementations: Entities have identifiers. Tables (or relations) have keys. We often use the terms interchangeably.

Relationships Entities are associated with one another in relationships: Relationship classes: associations among entity classes Relationship instances: associations among entity instances A relationship class can involve one, two or more entity classes. Unary, binary, ternary etc.

Binary Relationship

Ternary Relationship PRESCRIPTION DOCTOR MEDICINE PATIENT

Relationship Cardinality Maximum cardinality the maximum number of entity instances that can participate in a relationship with an instance of the related entity. 1-1, 1:M, M:N Minimum cardinality the minimum number of entity instances that must participate in a relationship with an instance of the related entity. 0 or 1, optional or mandatory

The Three Types of Maximum Cardinality

The Three Types of Minimum Cardinality

Data Modeling Notation

Data Modeling Notation: N:M and O-M Book says that ERwin does not indicate minimum cardinalities on N:M relationships But, this gets the point across

ID-Dependent Entities An ID-dependent entity is an entity whose identifier depends upon another entity. It’s composite. The ID-dependent entity is a logical extension of the other entity: BUILDING : APARTMENT PAINTING : PRINT The minimum cardinality from the ID-dependent entity is always one.

ID-Dependent Entities A solid line indicates an identifying relationship

A dashed line indicates a nonidentifying relationship Weak Entities A weak entity is an entity whose existence depends upon another entity. There is no specific notation for a nonidentifying but weak entity relationship A dashed line indicates a nonidentifying relationship

ID-Dependent and Weak Entities

Supertype & Subtype Entities Supertypes/subtypes are used when an entity set has subsets that: have special attributes, or participate in special relationships Suppose the Employee entity contains 3 types of employees: Hourly have an hourly rate Salaried have annual salary & stock options Consultants have contract # & billing rate All Employees have ID#, Name, Phone, Email, Hire date

Supertype & Subtype Entities

Exclusive or Inclusive Subtypes If subtypes are exclusive, one supertype relates to at most one subtype. Also called disjoint If subtypes are inclusive, one supertype can relate to one or more subtypes. Also called overlapping Also, some entity instances may not belong to any subtype. Discriminators are used in the to denote which subtype, if any, an instance is

Exclusive or Inclusive Subtypes examples

Subtypes denote “IS-A” relationships Relationships connecting supertypes and subtypes are called IS-A relationships because a subtype IS A supertype. The identifer of the supertype and all of its subtypes must be identical Subtypes are used to avoid value-inappropriate nulls, and to model special relationships

ERwin Symbol Summary

ERwin Symbol Summary (Continued)

David M. Kroenke’s Database Processing Fundamentals, Design, and Implementation (10th Edition) End of Presentation: Chapter Five Part One