database systems-lec3
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Chapter 5Entity Relationship (E-R)Modeling
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Basic Modeling Concepts Database design is both art and science.
A data model is the relatively simplerepresentation, usually graphic, of
complex real-world data structures. Itrepresents data structures and theircharacteristics, relations, constraints, andtransformations.
The database designer usually employsdata models as communications tools tofacilitate the interaction among thedesigner, the applications programmer,
and the end user.
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Data Models: Degrees of Data
Abstraction
Figure 2.10 page 57
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The conceptual model represents a global viewof the data. It is an enterprise-widerepresentation of data as viewed by high-levelmanagers.
Entity-Relationship (E-R) model is the most
widely used conceptual model.
The conceptual model forms the basis for theconceptual schema.
The conceptual schema is the visualrepresentation of the conceptual model.
The conceptual model is independent of bothsoftware (software independence) andhardware (hardware independence).
The Conceptual Model
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Tiny College Entities
Figure 2.11 Page 58
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A Conceptual Schema for Tiny College
Figure 2.12 Page 59
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The Internal Model
The internal model adapts theconceptual model to a specific DBMS.
The internal model is software-dependent.
Development of the internal model is
especially important to hierarchical andnetwork database models.
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Figure 2.13 Page 61
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Figure 2.13 Page 61
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The external model is the end usersview of the data environment.
Each external model is thenrepresented by its own externalschema.
CREATE VIEW CLASS_VIEW ASSELECT (CLASS_ID, CLASS_NAME,PROF_NAME, CLASS_TIME, ROOM_ID)FROM CLASS, PROFESSOR, ROOMWHERE CLASS.PROF_ID =
PROFESSOR.PROF_ID AND CLASS.ROOM_ID= ROOM.ROOM_ID;
The ExternalModel/Schema
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External
Model
Extern
alModel
Figure 2.14 Page 63
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Advantages of Using External Schemas
It makes application programdevelopment much simpler.
It facilitates the designers task bymaking it easier to identify specificdata required to support each businessunits operations.
It makes the designers job easier byproviding feedback about theconceptual models adequacy.
It helps to ensure security constraints
The ExternalModel/Schema
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The physical model operates at thelowest level of abstraction, describing theway data is saved on storage media suchas disks or tapes.
It requires the definition of both thephysical storage devices and the accessmethods required to reach the datawithin those storage devices.
The physical model is both software andhardware-dependent.
It requires detailed knowledge of
hardware and software used to
The Physical Model
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Entity Relationship (E-R)
Model
E-R model is commonly used to:
Translate different views of dataamong managers, users, andprogrammers to fit into acommon framework.
Define data processing andconstraint requirements to helpus meet the different views.
Help implement the database.
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E-R Model Components
Entities
In E-R models an entity refers to the entity set.
An entity is represented by a rectangle
containing the entitys name.
Attributes
Attributes are represented by ovals and areconnected to the entity with a line.
Each oval contains the name of the attribute it
represents. Attributes have a domain -- the attributes set of
possible values.
Attributes may share a domain.
Primary keys are underlined.
Relationships
Entity Relationship (E-R)
Model
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Attributes of the STUDENT Entity
Figure 4.1 Page 125
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Basic E-R Model Entity Presentation
Figure 4.3 Page 127
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The CLASS Table (Entity)
Components and Contents
Figure 4.2 Page 126
Cl f
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A simple attribute cannot be subdivided.
Examples: Age, Sex, and Maritalstatus
A composite attribute can be furthersubdivided to yield additional attributes.
Examples:
ADDRESS - Street, City, State, Zip
PHONE NUMBER - Area code,Exchange number
Classes ofAttributes
Classes of
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A single-valued attribute can have only asingle value.
Examples:
A person can have only one socialsecurity number.
A manufactured part can have only oneserial number.
Multivalued attributes can have manyvalues.
Examples:
A person may have several collegedegrees.
A household may have several phones
with different numbers
Classes ofAttributes
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Splitting the Multivalued Attributes
into New Attributes
Figure 4.4 Page 128
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A New Entity Set Composed of
Multivalued Attributes Components
Figure 4.5 Page 129
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A derived attribute is not physically storedwithin the database; instead, it is derived byusing an algorithm.
Example: AGE can be derived from the data
of birth and the current date.
Emp_Age = Current Date - Emp_DOB
Derived Attributes
Figure 4.6 Page 130
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A relationship is an association betweenentities.
Relationships are represented by diamond-shaped symbols.
Relationship
Figure 4.7 Page 132
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A relationships degree indicates thenumber of associated entities orparticipants.
A unary relationship exists when anassociation is maintained within a singleentity.
A binary relationship exists when twoentities are associated.
A ternary relationship exists when threeentities are associated.
Relationship Degree
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Relationship Degree
Figure 4.16 Page 143
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The Implementation
of a TernaryRelationship
Figure 4.17 Page 144
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Connectivity
The term connectivity is used to describethe relationship classification (e.g., one-to-one, one-to-many, and many-to-many).
Connectivity
Figure 4.7 Page 132
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Cardinality
Cardinality expresses the specific numberof entity occurrences associated with oneoccurrence of the related entity.
Cardinality
Figure 4.7 Page 132
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If an entitys existence
depends on the existenceof one or more otherentities, it is said to be
existence-dependent.
ExistenceDependency
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Weak Relationship
Class will not exist if course does not exist.
Class depends on Course
Figure 4.9 Page 134
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CLASS is Optional to COURSE
Figure 4.12 Page 138
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COURSE and CLASS in a Mandatory
Relationship
Figure 4.13 Page 139
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Weak Entities A weak entity is an entity that
Is existence-dependent and
Has a primary key that is partially ortotally derived from the parent entityin the relationship.
The existence of a weak entity is
indicated by a double rectangle. Theweak entity inherits all or part of itsprimary key from its strongcounterpart.
Weak Entities
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A Weak Entity in an ERD
Figure 4.14 Page 140
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Weak Relationship Between
DEPENDENT and EMPLOYEE
Figure 4.15 Page 141
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Recursive Entities
A recursive entity is one in which arelationship can exist betweenoccurrences of the same entity set.
A recursive entity is found within aunary relationship.
Recursive Entities
Figure 4.18 Page 145
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1. James Ramirez is married to Louise Ramirez, who ismarried to
James Ramirez
2. Anne Jones is married to Anton shapiro, who is married to
AnneJones
Recursive Entities
Figure 4.19 Page 145
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A rotor assembly is composed of four 2.5cm washers,two cotter pins, one 2.5 cm steel shank, four 10.25cmrotor blades, and two 2.5 cm hex nuts
Recursive Entities
Figure 4.20 Page 146
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The Implementation of the M:N Recursive
PART Contains PART Relationship
Figure 4.21 Page 146
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Implementation of the M:N COURSE
Requires COURSE RecursiveRelationship
Figure 4.22 Page 147
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Implementation of the 1:M EMPLOYEE
Manages EMPLOYEE RecursiveRelationship
Figure 4.23 Page 147
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A composite entity is composed of theprimary keys of each of the entities to beconnected.
The composite entity serves as a bridgebetween the related entities.
The composite entity may containadditional attributes.
Composite Entities
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The M:N Relationship Between
STUDENT and CLASS
Figure 4.25 Page 149
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A Composite Entity in the ERD
Figure 4.26 Page 149
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Converting the M:N Relationship
Into Two 1:M Relationships
Figure 4.24 Page 148
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Entity Supertypes and Subtypes
Figure 4.27 Page 150
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The generalization hierarchy depicts the
parent-child relationship.
The supertype contains the sharedattributes, while the subtype contains
the unique attributes. A subtype entity inherits its attributes
and its relationships from the supertypeentity.
Entity Supertypes and Subtypes
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A Generalization Hierarchy
Figure 4.28 Page 151
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Entity Supertypes and Subtypes
The supertype entity set is usuallyrelated to several unique and
disjointed (nonoverlapping) subtypeentity sets.
The supertype and its subtype(s)maintain a 1:1 relationship.
Entity Supertypes and Subtypes
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The EMPLOYEE/PILOT
Supertype/Subtype Relationship
Figure 4.29 Page 152
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Entity Supertypes and Subtypes
The generalization hierarchy depictsthe parent-child relationship. (Figure4.34)
The supertype contains the shared
attributes, while the subtype containsthe unique attributes.
The supertype entity set is usuallyrelated to several unique and
disjointed (nonoverlapping) subtype
Entity Supertypes and Subtypes
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A Generalization Hierarchy
With Overlapping Subtypes
Figure 4.30 Page 153
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References
ROB, P. AND CORONEL, C., 2004, Database
Systems. 6th Ed., Thomson Course Technology