Data modeling using the Entity–
Relationship(ER) model:
Entity – Relationship(ER) Model
ER data model allows us to describe the data involved in a
real world enterprise in terms of objects and their
relationships and is used to develop an initial database.
An entity is an object in the miniworld.
An attribute of an entity can have a value from a value
set (domain).
Database design process can be divided into following
steps :
1. Requirement collection and Analysis
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Understanding what data is stored
What applications must be built on top
What operation are most frequent for performance requirements
User defined operation applied to database
Data flow diagram techniques for specifying functional requirement
2. Creating conceptual schema
• concise description of data requirements of user
• Include detail descriptions of entity types , relationships,constraints
• Enable DB to concentrate on specifying the properties of data,without concerned with
storage detail.
3. Logical design or data model mapping
• Implementation of data model
• Conceptual schema is transformed from high level data model into the implementation
data model
4. Physical design
• Internal storage(structures ,access path , indexes , file organization) of database files
are specified.
Entity-Relationship(ER) Model
The ER model is a high-level conceptual data
model.
The ER model was introduced by Peter Chen in
1976, and is now the most widely used
conceptual data model.
ER MODEL CONCEPT
TYPE OF ATTRIBUTES
Simple or atomic : Attribute that are
not divided.
Composite Attribute hierarchy
Single – valued : Attribute which has single value. Eg Age
ENTITY TYPE AND KEY ATTRIBUTE
Displaying an Entity type
Entity type CAR with two key
Relationship Types , Sets and Instances
Relationship
When an attribute of one entity type refers to another
entity type
Represent references as relationships not attributes
Relationship
A relationship is an association among several entities.
Entity does not exits in isolation.
They always exist in relationship.
Degree of relationship
It is the number of participating entities.
A relationship of degree 2 is called binary and 3 is ternary.
Relationships usually have certain constraints that limit the
possible combinations of entities that may participate in
relationship instances.
Type of constraints
There are two types of relationship constraints
i. Cardinality ratio
ii. Participation constraints
A relationship relates two or more distinct entities with a specific
meaning. For example, EMPLOYEE John Smith works on the
ProductX PROJECT or EMPLOYEE Franklin Wong manages the
Research DEPARTMENT.
Relationships of the same type are grouped or typed into a
relationship type.
For example, the WORKS_ON relationship type in which EMPLOYEEs
and PROJECTs participate, or the MANAGES relationship type in
which EMPLOYEEs and DEPARTMENTs participate.
The degree of a relationship type is the number of participating
entity types. Both MANAGES and WORKS_ON are binary
relationships.
Cardinality Ratio
It specifies the number of relationship instances that an entity can participate.
Cardinality identifies the maximum number of instances of an entity that are
related to one instance to another entity.
Cardinality ratios for binary relationship types are :
One-to –one(1:1)
An entity in one table associated with at most one entity in other table.
Eg: one person holds one job
One-to-many(1:N)
An entity associated with any number of entities in other table but entity in
other table can associate with at most one entity of first table.
An employee can work for only one department but many employee can
work in a department.
Many to one
An entity in table associated with at most one entity in other table.An
entity in other table can be associated with any number of entities in first
table.
Many to many
An entity in table can associated with any number of entities in another
table and vice versa.
Eg An employee can work for several projects and several employees can
work on a project .
Participation
It specifies whether the existence of an entity depends on its being
related to another entity via relationship type.
There are types :
i)Total Participation
Every entity in entity set must participate in relation.
Ex: A company policy states that every emp must work for a
department or an employee entity can exist only if it participates in
relationship instances. Total participation is called existence
dependency.
Represented by double line
ii) Partial Participation
It means that some or part of the set is related.
Ex : In emp & dept relationship – only one employee manages a
depart not all.
Represented by single line
Notations of E-R diagram
Example relationship instances of the WORKS_FOR relationship between
EMPLOYEE and DEPARTMENT.
Attributes of Relationship Types
Relationship types can also have attributes ,similar to those of
entity types.
Eg to record the number of hours per week that an employee
works on a particular project .
Entity – Relationship Diagram(ER Diagrams)
Express the logical structure of a database graphically.
It describes data as entities , relationships and attributes.
Weak Entity Types
A weak entity must participate in an identifying relationship type
with an owner or identifying entity type
Entity types may not have any key attribute of their own.
Weak entity type are identified by being related to specific
entities in combination with some of their attribute values.
Entities are identified by the combination of:
A partial key of the weak entity type
The particular entity they are related to in the identifying
entity type.
Example:
Suppose that a DEPENDENT entity is identified by the
dependent’s first name and birthdate, and the specific EMPLOYEE
that the dependent is related to. DEPENDENT is a weak entity
type with EMPLOYEE as its identifying entity type via the
identifying relationship type DEPENDENT_OF
Constraints on Relationships
Constraints on Relationship Types
( Also known as ratio constraints )
Maximum Cardinality
One-to-one (1:1)
One-to-many (1:N) or Many-to-one (N:1)
Many-to-many
Minimum Cardinality
(also called participation constraint or existence
dependency constraints)
Structural Constraint Min, Max
• A more complete specification of the
structural constraint on a relationship type can
be given by the integer pair (min, max), which
means an entity must participate in at least
min and at most max relationship instances.
Specified on each participation of an entity type E in a relationship
type R
Specifies that each entity e in E participates in at least min and at most
max relationship instances in R
Default(no constraint): min=0, max=n
Must have minmax, min0, max 1
Derived from the knowledge of mini-world constraints
Examples:
A department has exactly one manager and an employee can manage
at most one department.
Specify (0,1) for participation of EMPLOYEE in MANAGES
Specify (1,1) for participation of DEPARTMENT in MANAGES
An employee can work for exactly one department but a department
can have any number of employees.
Specify (1,1) for participation of EMPLOYEE in WORKS_FOR
Specify (0,n) for participation of DEPARTMENT in WORKS_FOR
Data Modeling Tools
A number of popular tools that cover conceptual modeling
and mapping into relational schema design.
Examples: ERWin, S- Designer (Enterprise Application
Suite), ER- Studio, etc.
POSITIVES: serves as documentation of application
requirements, easy user interface - mostly graphics editor
support
Problems with Current Modeling Tools
DIAGRAMMING
Poor conceptual meaningful notation.
To avoid the problem of layout algorithms and aesthetics of
diagrams, they prefer boxes and lines and do nothing more
than represent (primary-foreign key) relationships among
resulting tables.(a few exceptions)
METHODOLGY
lack of built-in methodology support.
poor tradeoff analysis or user-driven design preferences.
poor design verification and suggestions for improvement.