Conceptual Modelingwith ER Diagrams

Peter Chen

introduced ER Diagrams

E-R Modeling

E-R Modeling is a design methodology for modeling the – Entities– Relationships

using a diagram with specified shapes for entities, relationships, roles of entities in these relationships, and cardinality.

E-R Diagram Conventions

Symbol Meaning

Entity Type

Weak Entity Type

Relationship

Identifying Relationship

Attribute

ER-Diagram Conventions - CONTINUED

Symbol Meaning

Key Attribute

Multivalued Attribute

Composite Attribute

Entities, Relationships and Attributes

Employee

• A entity is a object or event which needs to be tracked. Attributes characterize entity sets and relationships. Relationships relate two entity sets

Employee ProjectAssigned

Name Date

SSNum

Name

Payroll Application

A contractor does custom work (projects). He keeps track of employee time for each project. Besides materials, the customer is charged by invoice for employee time on his job. To keep track of employee time, the employee submits a time sheet with entries of project number, hours, description. These are entered into the system along with the employee ID. The hourly rate for the employee is used for each entry to generate a labor charge. The labor charges are totaled for each project and used as a basis for the customer invoice.

Understanding the Requirements

Management

EndUsers

Analysts

Conference

• Management – know high level business rules

• End users – have used paper system or previous software system and have expectations

• Analysts – have understanding of what can / cannot be done and must requirements into system

ER Diagrams for Payroll

Step 1. Identify the Entity Sets with Attributes

• Customers : Name, Address,Phone,CustID• Projects : Name,Description,ProjID,Total,CustID• Employees : Name,SSNo,PayRate,EmpID• LaborCharges : EmpID,ProjID,Description,Hours,Amount

ER Diagrams for Payroll

Step 1. Identify Relationships between Entity Sets with any Attributes

• Customers Has Project … Project is Weak Entity Set• Employee Works_On Project• Employee Has_Labor_Charge For Project … Labor_Charge is Weak Entity Set

E-R Modeling – Entity Sets, Attributes, Relationships

Racquet Club Example

• A Racquet Club has members with one year or two year contracts.

• Members pay contract amount up front, quarterly, or monthly.

• Members may reserve a court for the next day.

E-R Modeling – Entity Sets, Attributes, RelationshipsRacquet Club Example Continued

• Entity Sets

– Members : Name, Start, Contract_Amount, Contract_Period, Initial_Balance, Member_ID

– Payments : PayerID, Amount, Paid_Date, Payment_ID

– Reservations : Member_ID, CourtNumber, Hour• Relations :

– Member - Makes – Payment– Member - Makes - Reservation

Data Modeling –

Racquet Club Example

Keys, Cardinality Constraints and Participation Constraints

• A key of a relation is a minimal set of attributes and roles which uniquely determine each entity.

• A cardinality constraint m..n constrains the number of times a single entity can participate in a role of a relationship : m <= times <= n.

• A participation constraint ensures that an entity will particpate at least this many times.

ER-Diagram Cardinality Conventions Symbol Meaning

Calculated Attribute

Cardinality Constraints and Participation Constraints

0 .. *

0 .. 1

1 .. *

1 .. 1

Cardinality and Participationin the E-R model.

1 .. 2 means that at least 1 d must participate in the A relation with C and that not more that 2 d’s can participate. If A is represented by a table, then there is at least one row and not more than 2 rows with a single c value.

Many-to-one, one-to-one, and many-to-many correspondences

Cardinality & Participation Constraints

Library Circulation

Motel/Hotel

Video Store

Employment Company

Cardinality & Participation Constraints

Library Circulation

Book is_checked_out to Patron

Motel/Hotel Customer rents a room

Video Store Customer rents a video

Employment Company

Applicant is hired for a job

Example of an E-R diagram with an ISA hierarchy.

Using IsA for data partitioning.

New York Customer Pennsylvania Customer

Participation Constraints

A participation constraint is a lower bound on the number of times an entity can particpate in a relation.

Examples:

An employee belongs to ONE department. This is a participation and a cardinality constraint 1 .. 1.

Employee Department

0 .. *

0 .. 1

1 .. *

1 .. 1

Participation constraints.

Relationship Types – Two Entity Set Relations

A customer will pay for

at least one project

ProjectAssignedTo

Employee

Num-Hours

ProjectWill PayFor

Fixed-Cost

Customer

An employee is assigned to at least one project (participation)

Relationship Types – One & Three Entity Set Relations

One Set : ReportsTo( SupID, SubID )

Three Sets :

Sale( ProdID, CustID,SuppID;Date)

Employee Reports_To

subordinate

supervisor

Customer Product

Supplier

Sold

Date Price

ER Diagrams for Payroll

Customer

Name

Address

Phone

CustID

Projects

Name

Description

Total

CustID

ProjID

Has

ER Diagrams for Payroll

Employee

Name

SSNo

PayRate

EmpID

TimeSheet

Year

Month

EmpID

TSID

Has

PayCheck

Has

Labor Charges

Has

TSID

. . .

NetPay

TSID

. . .

Amount

The IS-A Relationship

Employee

Is-A

SalariedEmployee

Hourly Employee

disjoint

Name SSNo

SalaryPayRate

Construct an ER Diagram

• Lawn Furniture Manufacturer

• Stores Information for Catalog of products and parts

• Stores Information for Manufacturing, including quantity on hand

• Identify product entities (picnic table and lawn chair) with attributes

• Identify relationships IsA and PartOf

The Part-Of Relationship

Picnic_Table Lawn_Chair

ProdNo ProdNo

Part_Of

OutDoorProducts

IsA

Top Base Seats

QuantityProdNo

Price Price

Price

From E-R Diagrams toRelational Database Schema

• Converting entities into relations (tables)– Each entity becomes a relation– Each attribute of the entity becomes an

attribute of that relation (column)

– If attributes K1 ... Kn form a key of the entity, then K1 ... Kn form a candidate key of the relation.

From ER Diagramsto Relational Database Schema

• Translating Entities

EmployeeName SSNo

TypeHireDate

Termination

Create Table Employee ( Name char(20), Type char(1), HireDate date,

Termination date, SSNo char(9),

Primary Key (SSNo) )

From E-R Diagrams toRelational Database Schema

• Representing relationships in the database schema– R is a relationship between entity set A and weak

entity set B• A becomes a relation with primary key K and B+K becomes

a relation with K being a foreign key – e.g. employees and dependents

– R is a relationship between entity sets A,B and C• A,B and C become relations with KA , KB , and Kc as keys. R

becomes a relation with foreign keys KA , KB , and Kc

From ER Diagramsto Relational Database Schema

• Translating Relationships

Customer Product

Supplier

Sold

SaleDate Price

Create Table Sold ( SaleDate Date, Price Currency, SaleID autonumber,

Customer Integer, Supplier Integer, Product Integer,

Primary Key (SaleID),

Foreign Key ( Customer ) References Customers( CustID),

Foreign Key ( Supplier ) References Suppliers( SuppID ),

Foreign Key ( Product ) References Products ProdID ) )

SaleID

Translate into Schema

Employee

Is-A

SalariedEmployee

Hourly Employee

disjoint

Name SSNo

SalaryPayRate

Translate Into Schema

Picnic_Table Lawn_Chair

ProdNo ProdNo

Part_Of

OutDoorProducts

IsA

Top Base Seats

QuantityProdNo

Price Price

Price