Database Systems
Instructor: Khwaja Bilal Hassan
Data Modeling Using the EntityRelationship (ER) Model
Overview of DB Design Process
Design Stages
Analyze User Requirement
Develop conceptual model
Map conceptual to logical model
Choose DBMS
Develop physical design
Implement System
Test System
Operational Maintenance
Slide 1-4
Using High-Level Conceptual Data
Models for Database Design
 Requirements collection and analysis
– Database designers interview prospective database
users to understand and document data
requirements
– Result: data requirements
– Functional requirements of the application
Using High-Level Conceptual Data
Models (cont’d.)
 Conceptual schema
– Conceptual design
– Description of data requirements
– Includes detailed descriptions of the entity
types, relationships, and constraints
– Transformed from high-level data model into
implementation data model
Using High-Level Conceptual Data
Models (cont’d.)
 Logical design or data model mapping
– Result is a database schema in implementation
data model of DBMS
 Physical design phase
– Internal storage structures, file organizations,
indexes, access paths, and physical design
parameters for the database files specified
Conceptual Schema
Logical Design/Model
Physical Design/Model
Data Modeling Using the
Entity-Relationship (ER) Model
 Entity-Relationship (ER) model
– Popular high-level conceptual data model
– A semantic data model used for graphical
representation of conceptual database design
 ER diagrams
– Diagrammatic notation associated with the ER
model
 Unified Modeling Language (UML)
Entity Types, Entity Sets, Attributes,
and Keys
 ER model describes data as:
– Entities
– Attributes
– Relationships
Simple Example of ER Diagram
Another Example of ER Diagram
A Sample Database Application
 COMPANY
–
–
–
–
Employees, departments, and projects
Company is organized into departments
Department controls a number of projects
Employee: store each employee’s name, Social
Security number, address, salary, sex (gender),
and birth date
– Keep track of the dependents of each employee
Entity Type
 A name /label assigned to objects/items that
exists in real world and that have similar
properties
 It could be person, place, event or even
concept
 Distinguish from other entity types based on
properties
Entity Instance, Entity Set
 Entity Set: A particular object belonging to
particular entity type
 Example:
– Entity type: Student
– Entity instance: Fatima Ali
– Entity Set: All students
Symbols
Regular Entity Type
Weak Entity Type
EMPPLOYEE
BOOK
NAME
NAME
DEPENDENTS
BOOKCOPY
Naming convention for entity type
 Singular name recommended
 Organization specific name
– e.g. customer/client is same term choose which
organization is used
 Use capital letter
 Abbreviation can be used, be consistent
Attributes
 Attributes
– Particular properties that describe entity
– Types of attributes:
•
•
•
•
•
Composite versus simple (atomic) attributes
Single-valued versus multivalued attributes
Stored versus derived attributes
NULL values
Complex attributes
Symbols for Attributes
 Each represented as an oval, linked with an ET
symbol
Symbols for Attributes
Simple
Composite
Multi-valued
Derived
Symbols for Attributes
Example of derived Attributes
Example of composite
Attributes
Example
Experience
empId
empName
EMPLOYEE
dateHired
Hobbies
street
address
houseNo
Initial Conceptual Design of the
COMPANY Database
Super key
Super Key is a set of attributes whose set of values can uniquely
identify an entity instance in the entity set.
 A Super Key can contains one or more than one attributes.
Super Key is the broadest definition of unique identifiers of an entity
in an entity set.
Super key
The combination of “SSN” and “Name” is a super key of the following entity set
customer.
Because:
The value of attributes “SSN” and “Name”, such as 558-36-1234 and
Susan, can uniquely identify that particular customer in customer entity set, which
is the pool of all customers.
 We are unsurprisingly very interested in the most economical combination(s) of
attributes that can uniquely identify any particular entity.
 Therefore, we introduce Candidate Key next.
SSN
Customer-street
Customer-city
Customer-name
customer
Candidate key
Candidate key is a set of one or more attributes whose set of values
can uniquely identify an entity instance in the entity set.
 Any attribute in the candidate key cannot be omitted without
destroying the uniqueness property of the Candidate key.
It is minimal Super Key.
Candidate key
Properties of Candidate
Key…..
 While most entity sets have only one candidate key,
some entity sets could have more than one candidate
key.
 Candidate key could have more than one attributes.
 In building a database in a database software, the
software will only allow to use one candidate key to be
the unique identifier of an entity for an entity set.
Candidate key
 Example:
 (SSN, Name) is NOT a candidate key, because taking out
“name” still leaves “SSN” which can uniquely identify an entity.
“SSN” is a candidate key of customer.
 Example: Both “SSN” and “License #” are candidate keys of
Driver entity set.
SSN
Customer-street
Customer-city
Customer-name
customer
 Overall, Super Key is the broadest unique identifier;
Candidate Key is a subset of Super Key; and
Primary Key is a subset of Candidate Key.
 In practice, we would first look for Super Keys. Then
we look for Candidate Keys based on experience
and common sense.
 If there is only one Candidate Key, it naturally will be
designated as the Primary Key.
 If we find more than one Candidate Key, then we
can designate any one of them as Primary Key.
Primary Key
The Primary Key is an attribute or a set of attributes that uniquely identify a specific
instance of an entity.
 Every entity in the data model must have a primary key whose values uniquely identify
instances of the entity.
Primary Key
Properties Of Primary
Keys....
To qualify as a primary key for an entity, an attribute must have the
following properties:
 It must have a non-null value for each instance of the entity
 The value must be unique for each instance of an entity
 The values must not change or become null during the life of each
entity instance
Primary and Foreign keys are the most basic
components on which relational theory is based.
 Each entity must have a attribute or attributes, the
primary key, whose values uniquely identify each
instance of the entity.
Every child entity must have an attribute, the foreign
key, that completes the association with the parent
entity.
Foreign key
A Foreign key is an attribute that completes a relationship by
identifying the parent entity.
Foreign keys provide a method for maintaining integrity in the data
(called referential integrity) and for navigating between different
instances of an entity.
Every relationship in the model must be supported by a foreign key.
Foreign Key
Properties Of Foreign
Key….
 Every dependent and category (subtype) entity in the model
must have a foreign key for each relationship in which it
participates.
 Foreign keys are formed in dependent and subtype entities by
migrating the entire primary key from the parent or generic
entity. If the primary key is composite, it may not be split.
Composite Key
When a primary key is created from a combination of 2 or more columns, the
primary key is called a composite key.
Each column may not be unique by itself within the database table but when
combined with the other column(s) in the composite key, the combination is
unique.
Keys, and Value Sets
 Key or uniqueness constraint
– Attributes whose values are distinct for each
individual entity in entity set
– Key attribute
• Uniqueness property must hold for every entity set
of the entity type
Key Attributes
An attribute or set of attributes to identify
an entity instance uniquely
Types
– Super key
– Candidate key
– Primary key
– Secondary and Alternate keys
Example of Key
StdId
StdName
Address
S1020
Suhail Dar
Mareer Hassan
MCS
4
S1038
Shoaib Baber
Model Town
BCS
3
S1015
Tahira Ejaz
Wah Cantt
MCS
2
S1018 Arif Mehmood Satellite Town
BIT
4
S1025
BCS
6
Suhail Shah
Garhi Shahoo
ClName CurSem
Simple or Composite Key
A key consisting of single attribute is called
simple key, e.g., StudID, itemNo
A key consisting of more than one attribute is
known as composite
key, like
{Program_Code,Course_Code}
Composite Key Example
OFFERING
ProgCode
CourseCode
MarksAlloc
CrHrs
MCS
DS
100
3
MCS
DBS
100
3
MBA
DBS
100
3
BCS
NW
100
3
Composite Key Example
StdId
StdName
Address
S1020
Suhail Dar
Mareer Hassan
MCS
4
S1038
Shoaib Baber
Model Town
BCS
3
S1015
Tahira Ejaz
Wah Cantt
MCS
2
S1018 Arif Mehmood Satellite Town
BIT
4
S1025
BCS
6
Suhail Shah
Garhi Shahoo
ClName CurSem
Super Key
Definition same as of key
For example, for EMPLOYEE and
STUDENT entity types EmpID and
StudID are the superkeys respectively.
Super Keys
StdId
StdName
Address
S1020
Suhail Dar
Mareer Hassan
MCS
4
S1038
Shoaib Baber
Model Town
BCS
3
S1015
Tahira Ejaz
Wah Cantt
MCS
2
S1018 Arif Mehmood Satellite Town
BIT
4
S1025
BCS
6
Suhail Shah
Garhi Shahoo
ClName CurSem
Candidate Key
A candidate key is the super key that does not
contain extra attributes. It might have more
than one attribute that uniquely identifies an
entity. e.g. {name, address}
Candidate Keys
A super key such that no proper
subset of its attributes is itself a
super key. e.g. {StudID, Major} is
not a candidate key because it
contains a subset, StudID, that is a
super key
CK Example
StdId
StdName
Address
S1020
Suhail Dar
Mareer Hassan
MCS
4
S1038
Shoaib Baber
Model Town
BCS
3
S1015
Tahira Ejaz
Wah Cantt
MCS
2
S1018 Arif Mehmood Satellite Town
BIT
4
S1025
BCS
6
Suhail Shah
Garhi Shahoo
ClName CurSem
Primary Key
A primary key is the main/chosen candidate key
from the possible set of candidate keys that is most
suitable for entity identification.
 It may be a single attribute or a composite key.
 None of its attributes can have NULL values.
 The other candidate keys called Alternate keys
provide another method of accessing records.
Need for Key
 Need for unique identification and access
 Secondary Keys: We access on something not
necessarily unique
Summary
 Basic ER model concepts of entities and their
attributes
– Different types of attributes
– Structural constraints on relationships
 ER diagrams represent E-R schemas