Conceptual Modeling
with 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
• A entity is a object or event which needs to be
tracked. Attributes characterize entity sets and
relationships. Relationships relate two entity
sets
Employee
Name
SSNum
Employee
Name
Project
Assigned
Date
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
Conference
Management
End
Users
Analysts
• 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, Relationships
Racquet 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 Participation
in 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
0 .. *
0 .. 1
1 .. *
1 .. 1
Department
Participation constraints.
Relationship Types – Two Entity Set Relations
An employee is assigned to at least one project (participation)
Employee
Assigned
To
Project
Num-Hours
A customer will pay for
at least one project
Customer
Project
Will Pay
For
Fixed-Cost
Relationship Types – One & Three Entity Set Relations
One Set : ReportsTo( SupID, SubID )
subordinate
Reports_To
Employee
supervisor
Three Sets :
Sale( ProdID, CustID,SuppID;Date)
Customer
Date
Sold
Supplier
Product
Price
ER Diagrams for Payroll
Phone
Name
Customer
CustID
Address
Has
Name
Description
Total
CustID
Projects
ProjID
ER Diagrams for Payroll
PayRate
Name
Employee
EmpID
SSNo
Has
Year
EmpID
TimeSheet
Month
TSID
Has
Has
TSID
TSID
...
Labor Charges
PayCheck
...
NetPay
Amount
The IS-A Relationship
Employee
Name
SSNo
Is-A
Salaried
Employee
Salary
disjoint
Hourly
Employee
PayRate
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
OutDoorProducts
IsA
Lawn_Chair
Picnic_Table
Price
ProdNo
Price
Part_Of
Top
Seats
Base
Price
ProdNo
Quantity
ProdNo
From E-R Diagrams to
Relational 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 Diagrams
to Relational Database Schema
• Translating Entities
Employee
Name
SSNo
Type
Termination
HireDate
Create Table Employee ( Name char(20), Type char(1), HireDate date,
Termination date, SSNo char(9),
Primary Key (SSNo)
)
From E-R Diagrams to
Relational 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 Diagrams
to Relational Database Schema
• Translating Relationships
Customer
SaleDate
Sold
Supplier
SaleID
Product
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 ) )
Translate into Schema
Employee
Name
SSNo
Is-A
Salaried
Employee
Salary
disjoint
Hourly
Employee
PayRate
Translate Into Schema
OutDoorProducts
IsA
Lawn_Chair
Picnic_Table
Price
ProdNo
Price
Part_Of
Top
Seats
Base
Price
ProdNo
Quantity
ProdNo