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Entity
Relationship
Diagram (ERD)
مدلسازی اطالعات سازمان
A database is a collection of data that is organized in such a manner that its contents can be readily accessed, managed and updated
An Entity Relationship model (ERM) may be used to aid the development of a
database .An ER model may be represented pictorially by ERDs
Introduction
What is Conceptual Database Design?
Process of describing the data, relationships between the data, relationships between the data, and the constraints on the data.
After analysis - Gather all the essential data required and understand how the data are related
The focus is on the data, rather than on the processes. The output of the conceptual database design is a Conceptual Data Model ( +
Data Dictionary)
Introduction
What is a Data Model?
A detailed model that captures overall structure of organizational data while being independent of any implementation considerations
Data Model
Rationale for using Data
Modelling?
Typically data modelling is carried out parallel with other requirements analysis and structuring steps during the systems analysis phase
During this phase a data model is developed for the current system
Following this, the data model supporting the scope and requirements of the proposed system is developed
Data Model
Entity-Relationship (ER) Modeling.
ER Modeling is a top-down approach to database design. Entity Relationship (ER) Diagram Data modelling involves examining the data objects in a system
and identifying the relationship between these objects Notation uses three main constructs
Data entities
Relationships
Attributes
Chen Model &
Crow’s Foot
Model
Entity Relationship
Two Styles of ERD’s
The two most popular approaches to
construct entity relationship diagrams are:
SSADM
Chen ERD
Visio provides stencils for constructing
ERD’s using these approaches
ERD
ERD Symbols
SSADM Chen
Entity
Attribute
Relationship
8
Department
Staff No.
employs
Staff No
Department
Department 2
2
employs
ERD
Person, place, object, event
or concept about which data
is to be maintained
named property or
characteristic of an
entity
Association
between the
instances of one or
more entity types
Represents a set or collection of
objects in the real world that
share the same properties
Chen Notation
EntityName Verb Phrase AttributeName
An Entity
An entity is a thing of interest to a system about which information is kept
For example in a Hospital Administration System, some likely entities would include: Patient, Doctor, Operation, Ward
Each of these things are of interest to the system and would have data stored about them
ERD
Attributes
A key attribute uniquely identifies a specific occurrence of an entity
This may also be referred to as a primary key
Examples of primary keys include:
ISBN
Student Number
Staff ID
ERD
Identifier Attributes
Candidate key
Attribute (or combination of attributes) that uniquely
identifies each instance of an entity type
Some entities may have more than one candidate
key
Ex: A candidate key for EMPLOYEE is Employee_ID, a second
is the combination of Employee_Name and Address.
If there is more than one candidate key, need to make a
choice.
Identifier
A candidate key that has been selected as the
unique identifying characteristic for an entity type
Referential Attributes
Name IdNum DeptID Email
Ali 105 LG [email protected]
Mary 106 IT [email protected]
John 107 ENG [email protected]
Lim 108 IT [email protected] Instance of Lecturer.
Referential attribute: Ties the lecturer entity to
another entity that is department.
Make Reference to another instance in another table
Relationships
Associations between instances of one or more entity types that is of interest
Given a name that describes its function.
• relationship name is an active or a passive verb.
Author Book
Relationship name:
writes
An author writes one or more books
A book can be written by one or more authors.
Degree of Relationships
Degree: number of entity types that participate in a
relationship
Three cases
Unary: between two instances of one entity type
Binary: between the instances of two entity types
Ternary: among the instances of three entity types
Cardinality and Connectivity
Relationships can be classified as either one – to – one
one – to – many
many – to –many
Cardinality : minimum and maximum
number of instances of Entity B that can
(or must be) associated with each
instance of entity A.
Connectivity
Cardinality and Connectivity
Professor Class teaches
A professor teaches class OR
A class is taught by professor
How Many??
Professor Class teaches
Connectivity
Chen Model
1 to represent one.
M to represent many
Crow’s Foot
many
One
One or many
1
M
Mandatory one , means (1,1)
Optional? – we’ll see after this
2.5 Binary Relationships
1:M relationship
Relational modeling ideal
Should be the norm in any relational database design
The 1: M relationship between PAINTER and PAINTING
The Implemented 1:M relationship between PAINTER and PAINTING
Binary Relationships
1:1 relationship
Should be rare in any relational database design
A single entity instance in one entity class is
related to a single entity instance in another entity
class
Could indicate that two entities actually belong in
the same table
The 1:1 Relationship Between PROFESSOR and DEPARTMENT
The Implemented
1:1 Relationship
Between
PROFESSOR
and
DEPARTMENT
Binary Relationships
M:N relationships
Must be avoided because they lead to data
redundancies.
Can be implemented by breaking it up to produce a
set of 1:M relationships
Can avoid problems inherent to M:N relationship by
creating a composite entity or bridge entity
This will be used to link the tables that were
originally related in a M:N relationship
The composite entity structure includes-as foreign
keys-at least the primary keys of the tables that
are to be linked.
The M:N Relationship Between STUDENT and CLASS
This CANNOT be implemented as shown next…..
Bowser
Smithson
Accounting 1 (ACCT-211)
Intro to Microcomputing (CIS-220)
Intro to Statistics (QM-261)
The tables have many redundancies!!
+ CLASS_CODE
CLASS_CODE
+ STU_NUM
Changing the M:N relationship to TWO 1:M relationships
Converting the M:N relationship into TWO 1:M relationships
Foreign keys reference the primary keys in the
other tables of which it has a relationship with
The database designer has 2 main options to
define a composite table’s primary key:
either
use the combination of those foreign
keys or create a new primary key.
Mandatory vs. Optional Cardinalities
Specifies whether an instance must exist or can be
absent in the relationship
Lecturer Class handles
A Lecturer may handle zero or many classes.
A class is handled by one and only one Lecturer.
Optional Mandatory
(0,N) (1,1)
Lecturer Class
(0,N) (1,1)
handles 1
M
How to Evaluate a Data Model?
A good data model has the
following:
Accuracy and completeness
Non redundancy
Enforcement of business rules
Data Reusability
Stability and Flexibility
Communication Effectiveness
Simplicity
A Common Mistake
Modeling the business processes or functions
instead of the data.
What data we want to keep??
We are interested in modeling the data,
NOT the processes or functions that use
or generate those data.
Example:
Member Books Searches
M N
Is this part of the data requirement?
Are we interested to know the books searched by the members?
If answer is NO, then DO NOT include that as a relationship.
Use other appropriate diagramming techniques to capture the business
processes such as Data Flow Diagram.
Do not mix up the use of ER Modeling with DFD.
Choosing Entities
Example
A sale always starts with a customer
receiving an estimate. The estimate then becomes an order. An order can be for
one or more stock items. Each stock
item belongs to a certain stock category
(e.g. taps, sinks, cupboards etc.)
ERD
An Attribute An attribute is an item of data held about an
entity
In computer systems an attribute is a field of
information
Example : Sales System
Entity : Customer
Attributes : Customer Name, Customer Address,
Customer Phone Number
ERD
For Example
In a Sales System, customer name and
address are attributes of the entity customer.
The primary key is likely to be Customer Code or Customer ID as the unique
identifier for each customer in the system.
ERD
Designing a System:
Data Flow Diagramming
External entities, process, data stores
Answers question: “How will the data be used?”
Entity-Relationship Diagramming (ERDs)
Depict entities and their relationships amongst each other
Answers question: “What data will be captured in the database, and how will the data relate to each other?”
DFD Vs. ERD
Example : Babysitter
Service
The AITP Service Club wants to run a
babysitting service. Customers call to
request a sitter and the Club Coordinator
assigns an employee to sit for the
customer from a list of employees
available for the particular day
requested.
ERD Example
Babysitting System
Data is used to:
Assign employee to job
Determine availability
Data to be captured in database:
Employee data
Job data
Customer data
ERD Example
Entity-Relationship Model
A logical representation of the data of an
organization or business area in graphical
form
ERM
ERD - Babysitter Service
Employee
Job
Customer
assigned to Related to
ERD Example
Babysitting System Employee
Attributes: EmpID, Name,
Address, Phone Number,
Available Hours, JobNo Job
Attributes: JobNo, JobDate,
JobTime, EmpID Customer
Attributes: CusNo, Name,
Address, Phone Number, JobNo
ERD Example
Entity Instance
An entity instance is a single occurrence of an
entity type.
An entity type is described just once in a data
model while many instances of that data type
may be represented by data stored in the system.
There is one EMPLOYEE entity type in most
organisations but there may be hundred or
thousands of instances of this entity type in the
company.
42
Choosing Entities
Example
A sale always starts with a customer
receiving an estimate.The estimate then
becomes an order. An order can be for
one or more stock items. Each stock
item belongs to a certain stock category
(e.g. taps, sinks, cupboards etc.)
ERD Example
From the narrative we can deduce:
An Estimate becomes and Order
An Estimate is issued to a particular Customer
An Estimate refers to a Stock Item(s)
An Order is placed by the Customer
An Order refers to a Stock Item(s)
A Stock Item refers to a particular Stock Category
ERD Example
Order System: ERD
Customer
Order
Stock Item
Stock Category
Estimate
Places
Refers to
Becomes
Issued to
Refers to
Refers to
ERD Example
Entity Primary Key Other Attributes
Estimate Estimate Number Date
Customer Number
Stock Item Code
Order Order Number Same as Estimate
Customer Customer Number Customer Name
Customer Address
Credit Limit
Stock Item Stock Item Code Description
Number in Stock
Supplier Code
Stock Category Category Code Category Description
46
ERD Example
Entity Instance
An entity instance is a single occurrence of an entity type.
An entity type is described just once in a data model while many instances of that data type may be represented by data stored in the system.
There is one EMPLOYEE entity type in most organisations but there may be hundred or thousands of instances of this entity type in the company.
ERD Example
ERD Relationships
A relationship is an association between two
entities that is important to the system.
We must also consider the extent to which each
entity is related to another.
ERD Example
The Cardinality of a
Relationship Cardinality defines the minimum and maximum
number of occurrences of one entity for a single occurrence of the other
From the previous example we can say that:
A customer can place several orders if they wish
But an order can only be placed by one customer
ERD Example
Cardinality
Cardinality is bi-directional
This means that it must be defined for
both directions of the entity relationship.
ERD Example
One-to-One Relationship
(1:1) A one-to-one relationship occurs where a single
occurrence of one entity relates to one occurrence
of a second entity.
Estimate Order
An estimate can only become one order and an order is derived from one estimate.
becomes
ERD Example
One-to-Many
Relationship(1:N)
One-to-Many relationships occur when a
single entity instance is related to many
occurences of a second entity.
Customer Estimate
One customer can be issued with many estimates (the relationship is described as one-to-many)
is issued with
ERD Example
Many-to-Many Relationship
(N:M)
Many-to-many relationships occur
where many instances of an entity
relate to many instances of a
second entity
These relationships usually contain a
hidden entity which should be
extracted
Usually N:M relationships are broken
into one or more pairs of one-to-
many relationships
ERD Example
N:M
Many estimates refers to many stock items
(or many stock items are referred to by
many estimates)
Estimate Stock Item Refers to
ERD Example
Sales ERD
Customer
Order
Stock Item
Stock Category
Estimate
Places
Refers to
Becomes
Issued to
Refers to
Refers to
ERD Example
Cardinality Optional Cardinality
Where the analyst considers whether an
entity occurrence at one end of the
relationship can ever be present in the
system without the presence of the
corresonding occurrence of the entity
at the other end of the relationship
Mandatory Cardinality
When the occurrence of the entity at
either end of the relationship must be
present in the system
ERD Example
Mandatory - Mandatory
Some relationships are mandatory and
some are optional
Doctor Patient Is responsible for
A doctor must be responsible for one or more patients and a patient must be registered with one and only one doctor
Registered with
ERD Example
Optional – Mandatory
Doctor PROJECT Is responsible for
A doctor may be responsible for one or more patients and a patiente must be registered with one and only one doctor
Registered with
ERD Example
Mandatory – Optional
A doctor must be responsible for one of
more patients and a patient may be
registered with one and only one doctor
59
Doctor Patient
Is responsible for
Registered with
Optional – Optional
A doctor may be responsible for one or
more patients and a patient may be
registered with one and only one doctor
Doctor Patient
Is responsible for
Registered with
ERD Example
Relationship Cardinality Graphic Cardinality
Exactly 1
0 or 1
1 or more
0 or many
More than 1 61
ERD Example
Note
|| denotes 1 and only
ERD Example
