Data Modeling with ER Diagrams What is an Entity-Relationship Model? How is an E-R model represented as an E-R diagram? How can a video store be modeled as an E-R diagram? What is a role in a relationship type? How can we record the history of video rentals? How are cardinalities and related to roles? How can we represent purchase orders? How can we represent time cards and pay statements?

Entity-Relationship Modeling An E-R model is a data model that includes  Entity classes  Attributes of each class  Relationship types between classes  Constraints Types of attributes Designation of key attributes Cardinalities of relationship types An E-R Model is typically represented graphically –E-R diagram, the technique we use  UML diagram, an emerging standard for specifying E-R models and software design

Entity Relationship Diagrams Sample diagram for entity class Customer

E-R Diagram for Class Video Sample diagram for entity class Video

Relationship Types Example of representing relationship type Owns between classes Store and Video

Constraints on Relationship Types Example of cardinality and participation constraints

Modeling Video Rentals Examples of current rentals and previous rentals Differences are in cardinalities and attribute names

Modeling Video Rentals as an Entity Class Diagram shows entity class Rental and its relationship types with Customer and Video Note cardinalities and participation constraints  A Rental entity cannot exist without being related to both a customer and a video Problem occurs because Rental has no key attribute

Weak Entity Classes A weak entity class is  An entity class with no key of its own  An entity class whose entities cannot exist without being related to other entities An identifying relationship type is  A relationship type that determines the keys of the weak entities

Roles in Relationship Types Each entity in a relationship plays a specific role in it Roles are particularly important in relationship types that relate an entity class to itself  Each person in an IsChildOf relationship is either a parent or a child

Employee Roles and Cardinalities Is this an accurate model of the relationships between employees and stores? For example, does this diagram answer these questions correctly?  Does a store have to have a manager?  Can an employee be the manager of more than one store?  What other questions can you ask about roles and cardinalities?

Purchase Orders What does the diagram below mean?  How do we buy many copies of a single movie?  Does each video have its own videoId before it is received?  What does it mean for an order of a video to have a quantity? The problem with the diagram is with class Video The diagram fails to distinguish between a movie and a copy of a movie (a video) We need to revise the diagram to show Movie as an entity class

Modeling Videos and Movies This diagram more accurately specifies the true nature of Video and Movie  It also has an accurate specification of PurchaseOrder

Time Cards and Pay Statements Classes TimeCard and PayStatement are weak Does this diagram show the correct identifying relationships and discriminators?  For example, can an employee have 2 time cards that begin at the same time but are at different stores?