Software Engineering Lecture 5 Entity Relationship Modelling EntityRelationship

Software Engineering Lecture 5 Entity Relationship Modelling

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 2

What is an entity? An entity is an object that can be identified in the users’ work environment and that users want to track. 3

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 4

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 5

Attributes Example: Project = (proj_name, location, budget, start_date, end_date) Represented as ellipses in an ER diagram budget location proj_name 6 Project start_date end_date

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 7

Relationship Sets • 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. 8 starts at Road Town ends at manager works for Employee 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 location Project Hires Contractor Address budget end_date start_date Contract number 9 end_date 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: • • 10 one to 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 11 Person owns ID Book Person owns Car Person owns House

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 12

ER Diagram Symbols Entity Set Attribute One-to-one link Many-to-many link Relationship 13

Example ERD budget start_date ID end_date surname first_name proj_name location Project employs Employee position year_joined eliminates start_date species Alien 14 area end_date year_left

ER Design: Where to Start • Identify entity sets • Describe the entity sets with attributes • Specify relationships and cardinalities 15

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. 16

1. Define Entities Division Department Employee 17

2. Define Relationships Division contains Department manages Employee 18

3. Define Cardinality Division contains Department manages Employee 19

Standard Notation A plain rectangle is used to represent the entity type. INVOICE A labeled line to represent the relationship. is sent by 20 A B 1: 1 A B 1: N

Diagram with Standard Notation Division contains Department manages Employee 21

Notation Examples COURSE has enrolled STUDENT ONE course has enrolled ONE or MORE students. ONE student is enrolled on ONE course. LOAN refers to BOOK ONE loan refers to ONE book is referred to ONE loan. 22

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 23 supplies PART

More Examples Customer Teachers 24 is associated can place teach Property Order Students

Example - Simple Hospital System WARD has assigned NURSE 25 DOCTOR accommodates cares for treats PATIENT AILMENT suffers from

College System DEPARTMENT is_in offers COURSE LECTURER teaches enrols STUDENT 26