Post on 03-Apr-2018
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Outline
Symbols used in ER Diagram Cardinality Of a relationship
ER Constraints
Keys. Single Value constraints.
Foreign-key, or referential-
integrity.Domain constraints.
General constraints. EER concepts
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Single-value constraints
Single-value constraintsThe value in a certain context be unique for anattribute. Keys are single-value constraints. AlsoSingle-value constraints can seen in many-onerelationships.
Ex: 1. In students entity set, st_id is a singlevalue constraint.
2. In courses entity set course_no is asingle value constraint.
3. In the employees and depts entity sets, the
relationship heads is many-one. Here foreach entry
in employees, there will be atmost oneassociated
entity(instance) in depts.
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Referential integrity constraints
Weak entity: A weak entitycan beidentified uniquely only by considering theprimary key of another (owner) entity. i.e.,a weak entity can not be identified on its
own. Owner entity set and weak entity set must
participate in a one-to-many relationshipset (one owner, many weak entities).
Weak entity setmust have totalparticipation in this identifying relationshipset.
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Referential integrityconstraints Example
There must be referential integrityconstraint from weak entity set to thesupporting entity sets.
employee dependenthas
Emp-id
Emp-name Dep-name
gender
relationship
Birth-date
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Referential Integrity Contd..
An integrity constraint specifying thatthe value of an attribute in one relationdepends on the value of a primary key
in the same or another relation. i.e., ina 1:m relationship, a value in thematching column on the many side
must correspond to a value in theprimary key for some row in therelation on the one side or be NULL.
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Domain constraint
Constraint specifying set of values orrange of values that an attributevalue should lie.
Ex: 1. gender should be either M orF
2. course_no should start with
AAOC, BITS, CS, IS..
3. room_no should lie between 1
to 300
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General constraints (assertions)
Constraints/arbitrary assertions thatare specified in the database.
Ex: 1. no_of_courses taken by astudent
in a semester.
2. no_of_sections for a course.
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The ER conceptual schema diagram for the COMPANY database
(0,1) (1,1) (0,N)
(1,1)
(1,N)
(1,1) (4,N)
(1,N)
(0,N)
(1,1)
(1,1)(3,N)
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Why EER
The ER modeling concepts are notsufficient for representing new databaseapplications, which have more complex
requirements than do the more traditionalapplications.
Additional semantic data modelingconcepts must be used to represent theserequirements as accurately and clearly aspossible.
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EER
EER model concepts includes
All modeling concepts of basic ER
Additional concepts: subclasses/superclasses
specialization/generalization
categories (UNION types)
attribute and relationship inheritance
These are fundamental to conceptual modeling
The additional EER concepts are used to modelapplications more completely and moreaccurately
EER includes some object-oriented concepts,such as inheritance
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EER Modeling
Sub classes, Super classes andInheritance
Specialization and Generalization
Constraints and Characteristics of
Specialization and Generalization
Modeling of UNION Types Using
Categories
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Sub classes and Super classes
EMPLOYE
E
SECRETARY
ENGINEER
TECHNICIAN
SALARIED_EMPLOYEE
HOURLY_EMPLOYEE
Every entity that is a member of one of thesesubgroupings is also an employee
Superclasses: EMPLOYEE
Subclasses: SECRETARY,ENGINEER,
TECHNICIAN,
SALARIED_EMPLOYEE,
HOURLY_EMPLOYEE
An entity type may have additional
meaningful subgroupings of its entities
d
d
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Example
SECRETARY
d
TECHNICIAN
ENGINEER
d
SALARIED_EMP
HOURLY_EMP
MANAGER
EMPLOYEE
MANAGES
PROJECT
BELONGS_TO
TRADE_UNION
WORKSDEPARTMENT
EMPLOYEE: WORKS
SECRETARY: WORKSTECHNICIAN: WORKSENGINEER: WORKSMANAGER: WORKS, MANAGESSALARIED_EMP: WORKSHOURLY_EMP: WORKS, BELONGS_TO
TypingSpeed TGrade
EngType
FnameLname SSN
Addr
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Why class/subclass relationships andspecializations
Certain attributes may apply to some butnot all entities of the super class. A subclass is defined in order to group the
entities to which these attributes apply.The members of the subclass may still share
the majority of their attributes with the othermembers of the super class.
EMPLOYEE (Name, SSN, BirthDate, Address)SECRETARY (Name, SSN, BirthDate, Address,TypingSpeed)
ENGINEER (Name, SSN, BirthDate, Address,EngineerType)
TECHNICIAN (Name, SSN, BirthDate, Address,TGrade)
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Why need class/subclass relationshipsand specializations
Some relationship types may be
participated in only by entities thatare members of the subclass.
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Sub classes vs. Super classes
The set of entities in each sub class is a subsetof the entities that belong to EMPLOYEE
Each is called a subclass of EMPLOYEE
EMPLOYEE is the super class for each of thesesubclasses
The relationship between a super class and anyone of its subclass is called a super class/subclass or class/sub class relationship.
e.g., EMPLOYEE/SECRETARY andEMPLOYEE/TECHNICIAN are two class/subclassrelationships.
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Properties of Super classes andSub classes
A member entity of the subclassrepresents the same real-world entityas some member of the super class.
The subclass member is the same as the
entity in the super class, but in a distinctspecific role.
When implementing a super class/subclassrelationship, a member of the subclassmay be represented as a distinctdatabase object a distinct record thatis related via the key attribute to its superclass entity.
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Properties of Super classes and Subclasses (cont.)
An entity CANNOT exist in the DB merelyby being a member of a subclass. It mustalso be a member of the super class.
An entity can be a member of more thanone subclass. Example: A salaried employee who is also an engineer belongs
to the two subclasses ENGINEER and SALARIED_EMPLOYEE
It is not necessary that every entity in asuper class be a member of somesubclass Example: A technical writer is an employee but does not
belong to any subclasses.
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Type inheritance
The type of an entity is defined by theattributes it possesses and therelationship types which it participates.
An entity that is a member of a subclassinherits all the attributes of the entity asa member of the super class, as well as allthe relationships in which the super classparticipates.
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Example
EMPLOYEE
SECRETARYTECHNICIAN
d
Fname
Lname
SSN
TypingSpeed
TGrade
ENGINEER
EngType
SECRETARYFname, Lname, SSN, Addr
TypingSpeedTECHNICIAN
Fname, Lname, SSN, Addr, TGrade
ENGINEERFname, Lname, SSN, Addr,EngType
Addr EMPLOYEEFname, Lname, SSN,Addr
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Example
SECRETARY
d
TECHNICIAN
ENGINEER
d
SALARIED_EMP
HOURLY_EMP
MANAGER
EMPLOYEE
MANAGES
PROJECT
BELONGS_TO
TRADE_UNION
WORKSDEPARTMENT
Entity Type: Relationship TypeEMPLOYEE: WORKSSECRETARY: WORKS
TECHNICIAN: WORKSENGINEER: WORKSMANAGER: WORKS, MANAGESSALARIED_EMP: WORKSHOURLY_EMP: WORKS, BELONGS_TO
TypingSpee
d TGradeEngType