ERD with complete knowledge

DATABASE ARCHITECTURE
External level – concerned with the way individual
users see the data
Conceptual level – can be regarded as a community
user view a formal description of data of interest to
the organization
Internal level – concerned with the way in which the
data is actually stored

 In 1967 Peter Chen developed the ER Model.
 It is a high level data model used for developing the
conceptual design of the database.
 ER diagram helps designers understand and specify the
desired components of database and the relationship
among them.
 It must be flexible enough so that it can be used and
understood in practically any environment where
information is modelled

ENTITY
 It is a real world item / concept that can exist on it’s own.
 It may be an object with physical existence (
person, house) or it may be an object with conceptual
existence (company ,job, university course)
ENTITY TYPE
 Entity type defines collection of entities that have same
attribute.
 Entity type in a database is defined by it’s name and
attribute.
 Entity instance is a single occurrence of an entity type.

ENTITY SET
 Collection of entities of a particular entity type in a
database at any point of time is called entity set
 Entity set is usually referred to by same name as the
entity type
Entity type
 Entity type is represented as rectangle enclosing the type
name which is singular noun.
STUDENT

ATTRIBUTE
 Attributes are the properties that describe the entities.
 Attribute names are enclosed by ovals and connected to
their entities by single line.
 Set of attribute values of a given attribute is the value set
or domain
EMPLOYEE
Ssn
Salary
Bdate

SIMPLE ATTRIBUTE
• Cannot be split in to further
attributes(indivisible)
• Also known as Atomic
attribute
• Ex: Ssn(Social Security
Number)
COMPOSITE ATTRIBUTE
• Can be divided in to smaller
subparts which represent
more basic attributes with
independent meaning
• Even form hierarchy
• Value of the composite
attribute is the
composition of the
constituent simple
attributes
• Ex: Address

SINGLE VALUED ATTRIBUTE
• Attributes having single
value for particular entity.
• Ex - Age
MULTI VALUED ATTRIBUTE
• Attribute having set of
values
• Denoted by double circled
oval
• Ex: Phone-number, College-
degree
DERIVED ATTRIBUTE
• Attribute values are derived
from another attribute.
• Denoted by dotted oval
• Ex - Age
STORED ATTRIBUTE
• Attributes from which the
values of other attributes
are derived
• Ex: Bdate

IDENTIFIER ATTRIBUTE
OR KEY ATTRIBUTE
• Data item that allows us to uniquely identify individual
occurrences or an entity type.Key
• Attribute or set of attributes that uniquely identify a
tuple.Superkey
• Minimal super key with the property of irredusability
and uniquenessCandidate key
• An entity type may have one or more possible candidate
keys, the one which is selected as primary key.Primary key
• candidate key that consisting of two or more attributesComposite key
• An attribute or set of attribute that matches the
candidate key or other or same relationForeign key
The name of each primary key attribute is underlined.

SIMPLE KEY ATTRIBUTE
STUDENT
Rollno Name
COMPOSITE KEY ATTRIBUTE
FLIGHT
Flightid
No-of-
passengers
Date
Flightno

CRITERIA FOR SELECTING IDENTIFIERS
Some entities have more than one candidate identifier, so the
following criteria should be used:
 Choose identifier that will not change in value over the life of
each instance of the entity type
 Choose identifier that is guaranteed to have valid values and
will not be null (or unknown). If composite, make sure all
parts will have valid values

 Avoid the use of intelligent identifiers whose structure
indicates classifications, locations or people that might
change. e.g. the first two digits of an identifier may
indicate a warehouse location, but such codes are often
changed as conditions change, which renders them
invalid.

EMPLOYEE
Ssn
Salary
Bdate
AgePhone-
no
Address
City
State
Street-
add
Street
Apartme
nt-no
multivalued
derived
Primary-key
stored
Composite-
hierarchy

 COMPLEX ATTRIBUTE
Composite and mutivalued attribute can also be nested
arbitrarily to form complex key.
Address-
Phone
Address
Phone
Areacode
Phoneno
State
Street-add
Street
Apartment
no
City
Zip

SHOULD ‘ADDRESS’ ATTRIBUTE BE REPRESENTED
AS SIMPLE ATTRIBUTE OR COMPOSITE?
 Composite attributes are useful to model situations in which
user refers to the composite attribute as a unit but other
times refer specifically to it’s components.
 If the composite attribute is to be referenced only as a
whole then there is no need to represent it in to component
attributes.
 Ex: if there is no need to refer to the individual components
of an address ( ZIP, code, street etc) then the whole address
can be designated as a simple attribute.

RELATIONSHIP TYPE
 When attribute of an entity refers to another entity type
there exists relationship
 Ex: attribute ‘Manager’ of DEPARTMENT refers to another
employee who manages the department.
 Attribute ‘Controlling-dep’t refers to another department
that controls the project.
 In ER diagram these references must be represented as
relationship and not attributes.

 Ex: Relationship ‘WORKS-FOR’ between EMPLOYEE and
DEPARTMENT associates each employee with the
department for he works.
 The relationship is often denoted by diamond symbol and
are usually verbs.
 Each relationship instance in relationship set WORKS_FOR
associates one EMPLOYEE entity and one DEPARTMENT
entity.
EMPLOYEE DEPARTMENT
WORKS-
FOR

DEGREE OF A RELATIONSHIP
 If there are two entity types involved it is a binary
relationship type
 If there are three entity types involved it is a ternary
relationship type
 It is possible to have a n-array relationship (quaternary)
SALESASSIST PRODUCTSELLS
CUSTOMER Binary-relationship
It is the number of entity types that participate in a relationship

 Unary relationships are also known as a recursive
relationship.
 It is a relationship where the same entity participates
more than once in different roles.
 In the example above we are saying that employees are
supervised by employees.
EMPLOYEE
SUPERVI
SION

CARDINALITY CONSTRAINTS
 If we have two entity types A and B, the cardinality
constraint specifies the number of instances of entity B
that can (or must) be associated with entity A.
 Four possible categories are
One to one (1:1) relationship
One to many (1:m) relationship
Many to one (m:1) relationship
Many to many (m:n) relationship
The number of instances of one entity that can or must be
associated with each instance of another entity.

 one-to-one
1 1 1
 one to many
N 1
 many-to-many
 M N
EMPLOYEE DEPARTMENTMANAGES
EMPLOYEE DEPARTMENT
WORKS-
FOR
EMPLOYEE PROJECT
WORKS-
ON

ROLE NAME
 Some entities participate more than once in a relationship
type in different roles.
 Ex: Employee plays the role of supervisor as well as
supervisee.
 If the participating entity types are distinct then there is
no need for role name else role name is a must.
Role name represents role that a participating entity from the entity
type plays in the relationship.

PARITICIPATION
CONSTRAINTS(OPTIONALITY)
 Specifies minimum number of relationship instances
each entity can participate in .
 This is called minimum cardinality constraint.
 Two type of the participation are : Total And Partial
Specifies if existence of an entity depends on it being related to
another entity via relationship.
PARTICIPATION
TOTAL
PARTIAL

 Ex: if company policy says that every employee must work for
the department then participation of employee in work-for is
total.

 Total participation is also called existence dependencies.
 Every entity in total set of employee must be related to a
department via WORKS-FOR
 But we can’t say that every employee must MANAGE a
department .
 Hence relationship is partial.
 Total participation is indicated by double line and partial
participation by single line.
EMPLOYEE DEPARTMENT
WORKS-
FOR 1N
Cardinality + Optionality = multiplicity

ATTRIBUTE OF RELATIONSHIP TYPE
 Relationship can also have attributes
 Ex: Hours for WORKS-ON relationship between
EMPLOYEE and PROJECT
EMPLOYEE PROJECT
WORKS-
ON
Hours

 Attributes of 1:1 or 1:N relationship can be migrated to
one of the participating entity types.
 Ex: Start-date attributes of MANAGES can be attribute of
either DEPARTMENT or EMPLOYEE though conceptually
it belongs to manages.
EMPLOYEE DEPARTMENTMANAGES
Start-date
11

 Because each EMPLOYEE MANAGES is a 1:1 relationship.
 So every DEPARTMENT /EMPLOYEE entity participate in
atmost one relationship instance.
 So value of the Start-date can be determined separately
either by participating DEPARTMENT entity or
participating EMPLOYEE entity.

 For 1:N relationship a relationship attribute can be
migrated only to entity type on N-side of relationship
 Start-date attribute here can added only to employee.
EMPLOYEE DEPARTMENT
WORKS-
FOR
Start-date
1N

 For M:N relationship types some attribute are determined
by the combination of the participating entities, not by a
single enity.
 Such attribute must be specified as the relationship
attributes
 Ex: No.of hours an employee works on is department is
determent is determined by the EMPLOYEE-PROJECT
combination.
EMPLOYEE PROJECT
WORKS-
ON
Hours
NM

WEAK ENTITY AND STRONG ENTITY
 Entity belonging to weak entity type is identified by being
related to specific entities from another entity type in
combination with one of their attribute value.
 We call this entity type as identifying or owner entity
type (Parent/Dominanat Entity type)
 The relationship that connects owner entity type to weak
entity is called Identifying relationship.
 The weak entities are also called as child entity type or
subordinate entity type
Entity type that doesn’t have a key attribute on it’s own is
called weak entity and the Regular entity types that have key
value is called strong entities.

Weak entities have always a total participating constraint
because they cannot be identified without an owner entity.
EMPLOYEE DEPENDENTSHAS
N1
Employee
Dependent-
name
Relationship

 But converse is not true.
 Every existence dependency does not has result is weak
entities.
 Ex: Driver license entity can’t exist without person entity
though it has it’s own key (license number)
 There is total participation but not weak entity
 Weak entity type normally has partial
key(descriminator)
Partial key are set of attributes that can uniquely identify weak
entities that are related to some owner entity

 Partial key attribute is denoted with underlined or dotted line.
 Weak entities can sometimes be represented as complex
attributes (composite and multivalued attributes)
 Choice of the representation is done by DB designer.
 If there are many attributes then we can better represent
them as weak entity.
 Also if it participates independently in any other
relationship type other than it’s own identifying
relationship then it should not be modeled as complex
attributes.

DESIGN CHOICE FOR ER CONCEPTUAL
MODEL
 It is very difficult to determine if a particular concept
must be represented as entity type or attribute or
relationship type
 Some important concepts to be remembered are:
A) Concept may be first modeled as an attribute and then
refined in to a relationship because it is determined that an
attribute is reference to another attribute . A pair of such
attribute that are inverses of one another are refined to as
binary relationship

EMPLOYEE DEPARTMENT
WORKS-
FOR
1N
EMPLOYEE Department
DEPARTMENT Employees
Can be refined to binary relation

 B) An attribute that exists in several entity types may be
elevated or promoted to an independent entity.
 Ex: Suppose several entity types in an UNIVERSITY
DATABASE such as student , instructor & course each has
an attribute named department . Then we can better keep
an entity named department with a single attribute
department name and relate it to 3 entity types student ,
instructor & course via appropriate relationship.

INSTRUCTOR COURSE
DEPARTMENT
STUDENT Dept-name
COURSE
Can be refined as
Dept-name
INSTRUCTOR Dept-name
STUDENT
WORKS
-FOR
REGISTE
RED
BELONG
S-TO

 Inverse refinement to previous case:
 If an entity type department exists in initial design
with a single attribute dept-name and is related to
only one other entity type student , in this case
department may be reduced as an attribute of the
student

ALTERNATE NOTATIONS
 We can also associate pair of integer (min,max) with each
participation of entity type in a relationship type. These
are termed structural constraints
0<=min<=max and max>=1
Min>0 implies total particiaption
Min=0 implies partial participation

CHOOSING BETWEEN BINARY AND
TERNARY RELATIONSHIP
Consider the ER diagram
PROJECTSUPPLIER
PART
SUPPLY
Partno
Sname
Pname
Qty

PROJECTSUPPLIER
PART
SUPPLI
ES
Partno
Sname Pname
Qty
 We can refine it to three binary relations
USESCAN
SUPPLY

 CAN_SUPPLY relationship between SUPPLIER and PART
include instances (s,p) whenever SUPPLIER s can supply
PART p(to any project).
 USES relationship between PROJECT and PART include
instances (j,p) whenever PROJECT j uses PART p.
 SUPPLIES relationship between SUPPLIER and PROJECT
include instances (s,j) whenever SUPPLIER s SUPPLIES
some PART to PROJECT j.
 The existence of relationship instances (s,p) , (j,p) , (s,j)
does not necessarily imply that the instance (s,j,p) exists
in ternary relationship supply because meaning is
different.

 Solution is to include the ternary relationship plus one or
more of the binary relationship if they represent different
meaning and all are needed by the application.
 So ternary relationship supply can be represented as the
weak entity type with three identifying relationship

PROJECTSUPPLIER
PART
Partno
Sname Pname
Qty
SP
SS SUPPLY SPJ
1
N
N
1
1N

DEALING WITH TERNARY RELATIONSHIPS
 Consider the below relationship. ]It is no longer clear which
sales assistant sold a customer a particular product.
 So try replacing the ternary relations hip with an entity type and
a set of binary relationships.
PRODUCT
SALES-
ASSISTANT
CUSTOMER
SELLS
SELLS SELLS

 The relationship sells can become the entity type sale .
 So a sales assistant can be linked to a specific customer
and both of them to the sale of a particular product.
PRODUCT
SALES-
ASSISSTANT
CUSTOMER
REQ
UEST
MA
KES
SALE INVO
LVES

REMOVING REDUNDANT RELATIONSHIP
 Entities can be related indirectly by two relationships.
A relationship is redundant if it can be completely
represented by alternate transitive relationships

CONFIRMING OPTIONALITY AND
CARDINALITY OF RELATIONSHIP
 Use an `entity set diagram' to show entity examples graphically
 Consider the example of `course is_studied_by student'.

 Use the diagram to show all possible relationship
scenarios.
 Go back to the requirements specification and check to
see if they are allowed.
 If not, then put a cross through the forbidden
relationships
 This allows you to show the cardinality and optionality of
the relationship

DERIVING THE RELATIONSHIP PARAMETERS
 To check we have the correct parameters (sometimes also
known as the degree) of a relationship, ask two questions:
1. One course is studied by how many students?
Answer = `zero or more'.
 This gives us the degree at the `student' end.
 The answer `zero or more' needs to be split into two parts.
 The `more' part means that the cardinality is `many'.
 The `zero' part means that the relationship is `optional'.
 If the answer was `one or more', then the relationship
would be `mandatory'.

2.One student studies how many courses?
Answer = Òne'
 This gives us the degree at the `course' end of the
relationship.
 The answer òne' means that the cardinality of this
relationship is 1, and is `mandatory'
 If the answer had been `zero or one', then the cardinality
of the relationship would have been 1, and be òptional'.

CONSTRUCTING AN ER MODEL
 Before beginning to draw the ER model, read the
requirements specification carefully.
 Document any assumptions you need to make.
1.Identify entities
• list all potential entity types. These are the
object of interest in the system. It is better to
put too many entities in at this stage and them
discard them later if necessary.

2.Remove duplicate entities
• Ensure that they really separate entity types or just
two names for the same thing
• Also do not include the system as an entity type
• e.g. if modelling a library, the entity types might be
books, borrowers, etc.
• The library is the system, thus should not be an
entity type.

3.List the attributes of each entity
• Ensure that the entity types are really needed.
• Are any of them just attributes of another entity
type?
• If so keep them as attributes a nd cross them
off the entity list.
• Do not have attributes of one entity as
attributes of another entity!

4.Mark the primary keys
• Which attributes uniquely identify instances of
that entity type?
• This may not be possible for some weak
entities.
5.Define the relationships
• Examine each entity type to see its relationship
to the others.

6.Describe the cardinality and optionality
of the relationships
• Examine the constraints betwee n participating
entities.
7.Remove redundant relationships
• Examine the ER model for redundant
relationships.
 ER modelling is an iterative process, so draw several
versions, refining each one until you are happy with it.
Note that there is no one right answer to the problem, but
some solutions are better than others!

EXAMPLE
 “A Country Bus Company owns a number of busses.
Each bus is allocated to a particular route, although some
routes may have several busses. Each route passes through
a number of towns. One or more drivers are allocated to
each stage of a route, which corresponds to a journey
through some or all of the towns on a route. Some of the
towns have a garage where busses are kept and each of the
busses are identified by the registration number and can
carry different numbers of passengers, since the vehicles
vary in size and can be single or double-decked. Each route
is identified by a route number and information is available
on the average number of passengers carried per day for
each route. Drivers have an employee number, name ,
address, and sometimes a telephone number.”

ENTITIES
 Bus - Company owns busses and will hold information
about them.
 Route - Buses travel on routes and will need described.
 Town - Buses pass through towns and need to know about
them
 Driver - Company employs drivers, personnel will hold
their data.
 Stage - Routes are made up of stages
 Garage - Garage houses buses, and need to know where
they are.

RELATIONSHIPS
 A bus is allocated to a route and a route may have several
buses.
Bus-route (m:1) - is serviced by
 A route comprises of one or more stages.
route-stage (1:m) comprises
 One or more drivers are allocated to each stage.
driver-stage (m:1) is allocated
 A stage passes through some or all of the towns on a
route.
stage-town (m:n) passes-through

 A route passes through some or all of the towns
route-town (m:n) passes-through
 Some of the towns have a garage
garage-town (1:1) is situated
 A garage keeps buses and each bus has one `home' garage
garage-bus (m:1) is garaged

ATTRIBUTES
 Bus (reg- no,make,size,deck,no-pass)
 Route (route-no,avg-pass)
 Driver (emp - no,name,address,tel-no)
 Town (name)
 Stage (stage - no)
 Garage (name,address)

ALTERNATIVE DIAGRAMATIC NOTATIONS
FOR ER MODELS

Displaying attributes
Symbols for entity type attribute and relationship

ENHANCED ER DIAGRAM
The EER diagram includes the concepts of
Subclass and super class
Specialization and Generalization
Categorization
Attribute and relationship inheritance.

SUBCLASS SUPERCLASS AND
INHERITANCE
 In many cases entity types has numerous sub grouping of
it’s entities that are meaningful and need to be
represented explicitly because of their significance to
database application.

 Here each of the sub groupings is called subclass of the
EMPLOYEE entity type.
 EMPLOYEE entity type is called super class for each of
these subclasses
 The relationship between super class and any one of it’s
subclass is called the super class/subclass or simply
class/subclass relationship.
Member entity of the subclass represents the same real world
entity as some member of superclass and the Subclass entity
has specific distinct role

 An entity that is a member of subclass is also a member of
super class and can optionally be member of any number
of subclasses.
 Important concept associated with subclass is type
inheritance.
 Subclass entity possess it’s own attributes as well inherits
all the attributes possessed by it’s superclass.
 The entity also inherits the relationships that superclass
participtes

SPECIALIZATION
 This entity type is called superclass of the specialisation.
 Set of subclasses that form specialisation is defined based
on some distinguishing characters of entities in the
superclass.
 Ex: Job type , method-of-pay
Specialisation is a process of defining set of subclasses of an
entity type.

 The subclass that defines a specialisation are attached by
line to circle to represent the specialisation,which is
connected to superclass.
 The subset symbol on each line connecting a subclass to
the circle indicates direction of the superclass/subclass
relationship.
 Attributes that apply only to entities of particular entity
types – specific/local attributes.
 Subclass also participates in specific relationships.

WHY INCLUDE SPECIALISATION CONCEPT?
1. Certain attributes may apply only to
specific entity.
• A subclass is defined to group the entities
to which the attributes apply.
2. Some relationship types may be
participated in by only specific entities.

IN SPECIALISATION:
1.Define a set of subclass of entity type
2. Establish additional specific attributes with each
subclass
3.Establish additional specific relationship types
between each subclass and other entity types or
other subclass

GENERALISATION
The reverse process of abstraction in which we suppress the
differences among several entity types, identify their common
features and generalize them in to single super class of which
original entity types are special subclasses.
CAR
Vehicle-
id
Price
Licens
e-no
Max-
speed
No-of-
pass TRUCK
Vehicle-id Price
License
-no
Tonnage
No-of-
axles

TRUCK
TonnageNo-of-axles
CAR
Max-
speed
No-of-
pass
Vehicle-
id
Price
Licens
e-no
VEHICLE
d

CONSTRAINTS ON SPECIALIZATION AND
GENERALIZATION
Predicate-defined
subclasses
• Determine
exactly those
entities that
will become
members of
each subclass
by a condition.
Attribute defined-
specialization
• Membership
condition for
subclasses on
same attribute
of the super
class
• Attribute is
called the
defining
attribute of the
specialization
• Example:
JobType
User-defined
subclass
• no condition
determines
membership
• Membership in
a subclass is
determined by
the database
users by
applying an
operation to
add an entity to
the subclass

 Hence, we have four types of specialization/generalization
based on the paticipation of superclass entities in sub classes:
 Disjoint, total
 Disjoint, partial
 Overlapping, total
 Overlapping, partial
Generalization usually is total because the superclass is derived
from the subclasses.

Example of disjoint partial Specialization

SPECIALIZATION / GENERALIZATION
HIERARCHIES, LATTICES
 A subclass may itself have further subclasses specified on it
 Forms a hierarchy or a lattice.
Hierarchy has a constraint that every subclass has only one
superclass (called single inheritance)
In a lattice, a subclass can be subclass of more than one
superclass (called multiple inheritance)

 In a lattice or hierarchy, a subclass inherits attributes not
only of its direct superclass, but also of all its
predecessor superclasses
 In specialization, start with an entity type and then define
subclasses of the entity type by successive specialization
(top down conceptual refinement process)
 In generalization, start with many entity types and
generalize those that have common properties (bottom
up conceptual synthesis process).
 A subclass with more than one superclass is called a
shared subclass

SPECIALIZATION / GENERALIZATION LATTICE
EXAMPLE (UNIVERSITY)

CATEGORIES (UNION TYPES)
 Super classes represent different entity types .
 Example:
Database for vehicle registration, vehicle owner can be a
person, a bank (holding a lien on a vehicle) or a company.
◦ Category (subclass) OWNER is a subset of the union of
the three super classes COMPANY, BANK, and PERSON
◦ A category member must exist in at least one of its
super classes
Model a single super class /subclass relationship with more
than one super class
This is difference from shared subclass, which is subset of the
intersection of its super classes (shared subclass member must
exist in all of its super classes).

CASE IN POINT: E-R MODEL FOR
ONLINE SALES FOR BIGHIT VIDEO
“ BigHit Video Inc. wants to create an information system
for online sales of movies in both DVD and videotape format.
People will be allowed to register as customers of the online site
and to update their stored information. Informa-tion must be
maintained about customers ’ shipping addresses, e-mail ad-
dresses and credit cards. In a single sale, customers will be
allowed to purchase any quantity of videos. The items in a
single sale will be shipped to a single address and will have a
single credit card charge A customer will be provided with a
virtual shopping cart to store items to be purchased. As each
item is selected, it is added to the shopping cart. When the
customer finishes shopping, he will be directed to a checkout
area where he can purchase all of the items in the shopping
cart. At this time, payment and shipping information is entered.
Once the sale is complete, the shopping cart will be deleted and
the customer will be sent a receipt by e-mail.”

ENTITY ATTRIBUTE CONSTRAINTS
1.Customer accountId Key
lastName Not null
firstName
shippingAddresses Multivalued
composite with
components name,
street, city,
state, zipcode
emailAddress
creditCards Multivalued
composite with
components type,
accountNumber ,
expiration
password Not null at least 6
characters
IDENTIFYENTITYANDATTRIBUTES

2. Movie Movieid Key
Title
Genre
Media Either “dvd” or
“videotape”
determines
subclass
3. DVD Languages Subclass of Movie
Videoformat
Captioning
4. Videotape Format Subclass of Movie
Soundtrack

5. Sale SaleId Key
Totalcost
DateSold
CreditCard Composite with
components type,
accountNumber ,
and expiration
6. ShoppingCart CartId key
DateCreated

RELATIONSHIPS
Relationsh
ip Type
Entity
Class
Entity
Class
Cardinali
ty Ratio
Attributes
Purchases Customer Sale one-to-
many
Includes Sale Movie many-to-
many
quantity
Selects Customer Shopping
Cart
one-to-
many
Includes Movie Shopping
Cart
many-to-
many
quantity

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