entity relationship (er) modeling the er model developing and er diagram extended er model entity...
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Entity Relationship (ER) Modeling
The ER Model
Developing and ER Diagram
Extended ER Model
Entity Integrity : Selecting Primary Key
Chapter 4
In this chapter, you will learn:
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The main characteristics of entity relationship components
How relationships between entities are defined and refined
and how those relationships are incorporated into the
database design process
How ERD components affect database design and
implementation
That real-world database design often requires the
reconciliation of conflicting goals
The Entity Relationship (ER) Model
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ER model forms the basis of an ER diagram
ERD represents conceptual database as viewed
by end user
ERDs depict database’s main components:
◦ Entities
◦ Attributes
◦ Relationships
Entities
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Refers to entity set and not to single entity occurrence
Corresponds to table and not to row in relational
environment
In both Chen and Crow’s Foot models, entity is
represented by rectangle containing entity’s name
Entity name, a noun, is usually written in capital letters
Attributes
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Characteristics of entities
In Chen model, attributes are represented by ovals and
are connected to entity rectangle with a line
Each oval contains the name of attribute it represents
In Crow’s Foot model, attributes are written in attribute
box below entity rectangle
Attributes (continued)
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DomainsChapter 3 | Entity Relationship (ER) Modeling Prepared by : Maizatul Akma bt Napiah
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Attributes have domain
Domain is attribute’s set of possible values
Attributes may share a domaina set of permitted values
- ex) domain of customer--name : set of all text stringsof a certain length
Domains are the set of allowable data values for a Column.For example, the FiveDigitZipCode Column on the customer
entity can be in the integer domain. As such, the database would not allow you to place values like 123.45 (floating point) or ABC (character) into that Column.
Identifiers (Primary Keys)
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Underlined in the ERD
Key attributes are also underlined in frequently
used table structure shorthand
Primary Key Guidelines
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Attribute or combination of attributes that
uniquely identifies entity instances in an entity
set
Main function is to uniquely identify an entity
instance or row within a table
Guarantee entity integrity, not to “describe” the
entity
Primary Key Guidelines (continued)
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Composite Primary Keys
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Primary keys ideally composed of only single
attribute
Possible to use a composite key
Primary key composed of more than one attribute
Composite Primary Keys (continued)
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When to Use Composite Primary Keys
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Useful as identifiers of composite entities, where
each primary key combination is allowed only
once in M:N relationship
◦Automatically provides benefit of ensuring that there
cannot be duplicate values
When to Use Composite Primary Keys(continued)
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When to Use Composite Primary Keys(continued)
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Useful as identifiers of weak entities, where
weak entity has strong identifying relationship
with parent entity
◦Normally used to represent:
A real-world object that is existent dependent on another
real-world object
A real-world object that is represented in data model as
two separate entities in strong identifying relationship
Composite and Simple Attributes
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Composite attribute can be subdivided
Simple attribute cannot be subdivided
Single-Valued AttributesChapter 3 | Entity Relationship (ER) Modeling Prepared by : Maizatul Akma bt Napiah
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Single-value attribute can have only a single value
Multivalued AttributesChapter 3 | Entity Relationship (ER) Modeling Prepared by : Maizatul Akma bt Napiah
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Multivalued attributes can have many values
Multivalued Attributes (continued)
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Resolving Multivalued Attribute Problems
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Although conceptual model can handle M:N
relationships and multivalued attributes, you should
not implement them in relational DBMS
◦ Within original entity, create several new attributes, one
for each of the original multivalued attribute’s
components
Can lead to major structural problems in table
◦ Create new entity composed of original multivalued
attribute’s components
Resolving Multivalued Attribute Problems (continued)
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Resolving Multivalued Attribute Problems (continued)
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Resolving Multivalued Attribute Problems (continued)
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Derived Attributes
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Attribute whose value may be calculated
(derived) from other attributes
Need not be physically stored within database
Can be derived by using an algorithm
Derived Attributes (continued)
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Derived Attributes (continued)
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Relationships
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Association between entities
Participants are entities that participate in a
relationship
Relationships between entities always operate in both
directions
Relationship can be classified as 1:M
Relationship classification is difficult to establish if
know only one side of the relationship
Connectivity and Cardinality
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Connectivity
◦ Used to describe the relationship classification
Cardinality
◦ Expresses minimum and maximum number of entity
occurrences associated with one occurrence of related
entity
Established by very concise statements known as
business rules
Connectivity and Cardinality (continued)
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Existence Dependence
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Existence dependence
◦ Exist in database only when it is associated with
another related entity occurrence
Existence independence
◦ Entity can exist apart from one or more related
entities
◦ Sometimes refers to such an entity as strong or
regular entity
Relationship Strength
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Weak (non-identifying) relationships
Exists if PK of related entity does not contain PK
component of parent entity
Strong (Identifying) Relationships
Exists when PK of related entity contains PK
component of parent entity
Weak (Non-Identifying) Relationships
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Weak (Non-Identifying) Relationships (continued)
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Strong (Identifying) Relationships
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Weak Entities
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Weak entity meets two conditions
◦ Existence-dependent
Cannot exist without entity with which it has a relationship
◦ Has primary key that is partially or totally derived
from parent entity in relationship
Database designer usually determines whether
an entity can be described as weak based on
business rules
Weak Entities (continued)
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Weak Entities (continued)
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Relationship Participation
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Optional participation
◦ One entity occurrence does not require
corresponding entity occurrence in particular
relationship
Mandatory participation
◦ One entity occurrence requires corresponding entity
occurrence in particular relationship
Relationship Participation (continued)
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Relationship Participation (continued)
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Relationship Participation (continued)
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Relationship Participation (continued)
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Relationship Degree
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Indicates number of entities or participants associated
with a relationship
Unary relationship
◦ Association is maintained within single entity
Binary relationship
◦ Two entities are associated
Ternary relationship
◦ Three entities are associated
Relationship Degree (continued)
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Relationship Degree (continued)
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Recursive Relationships
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Relationship can exist between occurrences of
the same entity set
Naturally found within unary relationship
Recursive Relationships (continued)
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Recursive Relationships (continued)
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Recursive Relationships (continued)
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Recursive Relationships (continued)
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Recursive Relationships (continued)
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Recursive Relationships (continued)
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Composite Entities
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Also known as bridge entities
Composed of primary keys of each of the entities
to be connected
May also contain additional attributes that play no
role in connective process
Composite Entities (continued)
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Composite Entities (continued)
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Composite Entities (continued)
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Developing an ER Diagram
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Database design is iterative rather than linear or
sequential process
Iterative process
Based on repetition of processes and procedures
Developing an ER Diagram (continued)
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Building an ERD usually involves the following activities:◦ Create detailed narrative of organization’s description of
operations◦ Identify business rules based on description of
operations◦ Identify main entities and relationships from business
rules◦ Develop initial ERD◦ Identify attributes and primary keys that adequately
describe entities◦ Revise and review ERD
Developinag an ER Diagram (continued)
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Tiny College◦Tiny College is divided into several schools
Each school is composed of several departments◦Each department may offer courses◦Each department may have many professors assigned to
it◦Each professor may teach up to four classes; each class
is section of course◦Student may enroll in several classes, but (s)he takes
each class only once during any given enrollment period
Developing an ER Diagram (continued)
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Tiny College (continued)◦Each department has several students
Each student has only a single major and is associated with a single department
◦Each student has an advisor in his or her department Each advisor counsels several students
◦The relationship between class is taught in a room and the room in the building
Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
Chapter 3 | Entity Relationship (ER)
Modeling
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Data Modeling Checklist
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Summary
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Entity relationship (ER) model
◦ Uses ERD to represent conceptual database as
viewed by end user
◦ ERM’s main components:
Entities
Relationships
Attributes
◦ Includes connectivity and cardinality notations
Summary (continued)
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Connectivities and cardinalities are based on
business rules
In ERM, M:N relationship is valid at conceptual
level
ERDs may be based on many different ERMs
Database designers are often forced to make
design compromises
Summary
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Extended entity relationship (EER) model adds
semantics to ER model via entity supertypes, subtypes,
and clusters
Specialization hierarchy depicts arrangement and
relationships between entity supertypes and entity
subtypes
Inheritance allows an entity subtype to inherit attributes
and relationships of supertype
Summary (continued)
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Composite keys are useful to represent M:N
relationships and weak (strong-identifying) entities
Summary (continued)
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Surrogate primary keys are useful when there is no
natural key that makes a suitable primary key
In a 1:1 relationship, place the PK of mandatory
entity as foreign key in optional entity
Time-variant data refers to data whose values
change over time and whose requirements mandate
that you keep a history of data changes
Summary (continued)
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Data modeling checklist provides way for
designer to check that the ERD meets set of
minimum requirements