Lecture 5 Entity Relationship Modeling Software Engineering Lecture 5 Entity Relationship Modeling

Entity-Relationship Model Technique for carrying out the conceptual and logical design of the system A widely accepted data modelling approach 3 basic notions: entities attributes relationships

What is an entity? An entity is an object that can be identified in the users’ work environment and that users want to track.

Entities An entity is a thing or object in the real world (within the application context) An entity has a set of properties which uniquely identify it. An entity is represented as a rectangle in an ER diagram Project

What is an Attribute? An attribute describes a characteristic of an entity For example: An entity: Employee Has attributes: Employee_Name Extension Date_Of_Hire

Attributes Example: Project = (proj_name, location, budget, start_date, end_date) Represented as ellipses in an ER diagram budget start_date location Project end_date proj_name

What are Relationships? Relationships are associations between entities which express some real world relationship Project Employs Employee Project and Employee participate in the employs relationship The function that an entity plays in a relationship is called that entity’s role

Relationship Sets starts at There can be more than one relationship between entities. There can be recursive relationships that can indicate roles for clarity. A relationship can also have descriptive attributes. Road Town ends at manager Employee works for worker date Client orders Book

Degree of Relationships Refers to the number of entities participating in a relationship Most relationships are binary (degree 2), but can also have ternary (degree 3) relationships. Name proj_name Project Hires Contractor location Address budget end_date start_date end_date Contract number start_date value

Mapping Cardinalities Express the number of entities to which another entity can be associated via a relationship For a binary relationship, the mapping cardinality can be one of the following: one to one one to many many to one many to many

Cardinality Examples A person has only one ID book and an ID book belongs to only 1 person A person can own many cars (possibly 0), but a car only has 1 owner A person can own many houses and a house can have multiple owners Person ID Book owns Person Car owns Person House owns

Design Issues Entity or Attribute? Example: should location be an entity or an attribute? can a project be in more than 1 location? Entity or Relationship? generally: relationships for actions between entities whatever “makes sense” in application context

ER Diagram Symbols Entity Set One-to-one link Many-to-one link Attribute Many-to-many link Relationship

Example ERD Project Employee Alien employs eliminates start_date budget ID surname end_date first_name proj_name Project employs Employee location position year_joined start_date end_date eliminates year_left species Alien area

ER Design: Where to Start Identify entity sets Describe the entity sets with attributes Specify relationships and cardinalities

Simple Modelling Example Every department within the company is in only one division. Each division has more than one department in it. There is no upper limit on the number of departments that a division can have.

1. Define Entities Division Department Employee

2. Define Relationships Division contains Department manages Employee

3. Define Cardinality Division contains Department manages Employee

Standard Notation A plain rectangle is used to represent the entity type. INVOICE A labeled line to represent the relationship. is sent by 1 : 1 A B A B 1 : N

Diagram with Standard Notation Division contains Department manages Employee

Notation Examples ONE course has enrolled ONE or MORE students. ONE student is enrolled on ONE course. LOAN BOOK refers to ONE loan refers to ONE book. ONE book is referred to ONE loan.

Many-to-Many Relationship Example Any one supplier might supply more than one kind of part. Any one kind of part might be bought from a number of different suppliers. SUPPLIER PART supplies

More Examples is associated can place teach Customer Property Customer Order teach Teachers Students

Example - Simple Hospital System WARD DOCTOR has accommodates assigned treats NURSE PATIENT AILMENT cares for suffers from

College System DEPARTMENT COURSE LECTURER STUDENT is_in offers teaches enrols STUDENT

Your Project Work Develop Requirements Document. Think about your system’s entities and relationships