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More on ER Model Detail Relationships

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Attributes

Attribute A property or characteristic of an entity type

that is of interest to the organisation Simple Attribute

An attribute that cannot be broken down into smaller components

e.g. Emp No

Emp No

Detail Conceptual Data Model

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Multi-valued Attribute An attribute that may take on more than one

value for a given entity instancee.g. Employee Skills, Qualifications

Attributes Cont’d

Composite Attribute An attribute that can be broken down into

component parts

e.g. Address (Street, City, State, Postal Code) Name (First Name, Middle Initials, Last

Name)

Skills

Name

First Name

Mid Initials

Last Name

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Attributes Cont’d

Stored Attribute An attribute whose valued is stored in the

database Derived Attribute

An attribute whose values can be calculated from related attribute values

e.g. Years Employed (using Employed Date) Age (using Date of Birth)

Age

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Identifier

Composite Identifier An identifier that consists of a composite

attributee.g. Flight Id (Flight No, Date)

Employee Emp No

Flight Flight Id

Flight No

Date

Identifier An attribute (or combination of attributes)

that uniquely identifiers individual instances of an entity type

e.g. Emp No

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Identifier Choose an identifier that will not change its

value over the life of each instance of the entity type

Choose an identifier such that each instance of the entity type, the attribute is guaranteed to have valid values and not be null (or unknown)

Avoid the use of so-called intelligent identifiers, whose structure indicates classifications, etc.

Consider substituting single-attribute identifiers for large composite identifiers

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Attributes

DepartmentDepartment

NumberLocation

NamePhone

Manager

Start date

Control ProjectsEmployees work for

Number of Employees

Detailed Conceptual Design

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Dept No unique identifier of a dept. IdentifierDept Name name of a department UniqueLocation location of a department Multi-valued Phone phone no. of a departmentEmployees no. of employees in a dept. Derived

Dept No

Dept Name

Location

Employees

Phone

Department

Detailed Conceptual Design

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Detailed Conceptual Design

ProjectProject

NameNumberLocation Leader Employee

Department Control

Proj No unique identifier of a project IdentifierProj Name name of a project UniqueLocation location of a project

Project

Proj No

Proj Name

Location

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Detailed Conceptual Design

EmployeeEmployee

NameNational IDAddressSalarySexBirth Date

Works for DepartmentSupervise Employee

Emp No

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Detailed Conceptual Design

EmployeeEmployee

Emp No unique identifier of an emp. IdentifierEmp Name name of an employee Composite First Name first name of an employee Mid Initials middle initials of an employee Last Name last name of an employeeNID national id of an employee UniqueAddress address of an employeeSalary salary of an employeeGender sex of an employeeDOB birth date of an employee

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Detailed Conceptual Design

Emp No

Emp Name

NID

Salary

Address

Employee

GenderDOB

First Name

Mid Initials

Last Name

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Detailed Conceptual Design

DependentDependent

Name Sex Birth Date Relationship

Depd Name name of a dependent Part of KeyGender sex of a dependent DOB birth date of a dependentRelation relationship of a dependent to an employee

Dependent

Depd Name

Gender

DOB

Relation

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Employee

works

Department

Project

manage

controlworks on

supervise

Dependent

has

Depd Name

GenderDOBRelation

Emp No

Emp NameNID

Salary

Address

Gender

DOB

First Name

Mid Initials

Last Name

Proj No

Proj NameLocation

Dept No

Dept Name

Location

Employees

Phone

Hours

Start d

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Entity Types

Weak Entity An entity types whose existence depends on

some other entity

Dependent

Employee

Strong (Regular) Entity An entity that exists independently of other

entity types

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Entity Types

Identifying Relationship A relationship between a weak entity type

and its owner

Identifying Owner The entity type on which the weak entity

type dependse.g. Employee is the Owner of Dependent

has

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Employee

works

Department

Project

manage

controlworks on

supervise

Dependent

has

Conceptual Design showing weak entities

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Sample Entity Definitions

Name: DepartmentType: RegularDefinition: a department of an organisationIdentifier: Dept_No

Name: DependentType: WeakDefinition: a person who is a dependent of an employee and entitle for insuranceIdentifier: Depd_Name (partial only)

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Sample Attribute Definitions

Name: Emp_NoDomain: employee identitiesDefinition: unique identifier of an employeeNull: No

Name: Emp_NameComponents: First_Name, Mid_Initials,

Last_NameDefinition: a partial identifier of a nameNull: No

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Sample Relationship Definitions

Name: Works_forType: binary 1:MDefinition: associates each employee with a

departmentConstraint: each employee must be attached to a

departmentAttributes: none

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Sample Domain Constraints

Name: Employee identitiesData Type: characterLength: 5Allowable Characters: digits

Domain Constraints A specification of the characteristics of the

data values that can be associated with one or more attributes

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Sample Domain Constraints Cont’d Name: Last name

Data Type: characterMax Length: 20

Name: DOBData Type: dateFormat: dd/mm/yyyy

dd = daymm = monthyyyy = year

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Associative Entity

An entity type that associates the instances of one or more entity types and contains attributes that are peculiar to the relationship between those entity instances

Certificate

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Completes

Date Comp

Course

CNameCourse Id

Employee

Emp NameEmp No

Certificate

Course

CNameCourse Id

Employee

Emp NameEmp No Date Comp

Cert No

4

4

30

30

11 many to may relationship many to may relationship

22 one to many relationships one to many relationships

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Associative Entity All of the relationships for the participating

entity types are “many” relationships The resulting associative entity type has

independent meaning to end users, and preferably can be identified with a single-attribute identifier

The associative entity has one or more attributes, in addition to the identifier

The associative entity participates in one or more relationships independent of the entities related in the associated relationships

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Relationships Unary Relationship

A relationship between the instances of a single entity type

e.g. Person is married to a Person (1:1) Employee manages Employees (1:M)

Binary Relationship A relationship between the instances of two entity

types

Employee

manages

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Relationships Ternary Relationship

A simultaneous relationship among the instances of three entity types

S1 P1 W1 Land 10S1 P1 W2 Sea 15S1 P2 W1 Air 20S2 P1 W1 Air 15

Part

Warehouse

SuppliesVendor

Shipping Mode

Unit Cost

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Relationships Ternary Relationship

can be treated as two many to many relationships

Part

Vendor

WarehouseSupplies

Shipping Mode

Unit Cost

supply

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Entity Relationship Model

studno name

given family

hons

slot

labmark

exammark

STUDENT

SCHOOL

YEARENROL

YEARREG

REG

TUTOR

YEARTUTOR

STAFFCOURSE

courseno

subjectequip

name

year

faculty

appraiserappraisee

APPRAISAL

TEACHm n

1 m

1

11m

n mm

1

m

1

roomno

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Superclasses, Subclasses; Specialisation & Generalisation Relationships

Subclasses and Superclasses a subclass entity type is a specialised type of

superclass entity type a subclass entity type represents a subset or

subgrouping of superclass entity type’s instances e.g. undergraduates and postgraduates are

subclasses of student superclass Attribute Inheritance

subclasses inherit properties (attributes) of their superclasses

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Constraints on Specialisation & Generalisation

Specialisation the process of defining a set of more specialised entity types of

an entity type Generalisation

the process of defining a generalised entity type from a set of entity types

Predicate/Condition defined determine the entities that will become members of each

subclass by a condition on an attribute value. All member instances of the subclass must satisfy the predicate

e.g. first years and second years are subclasses of undergraduates based on their year attribute.

User defined no condition for determining subclass membership

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Constraints on Specialisation & Generalisation

Disjointness Overlap

• the same entity instance may be a member of more than one subclass of the specialisation

Disjoint• the same entity instance may be a member of only one subclass of the

specialisation

Completeness Total

• every entity instance in the superclass must be a member of some subclass in the specialisation

Partial• an entity instance in the superclass need not be a member of any

subclass in the specialisation

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Specialisation & Generalisation Relationships

name

given family

STUDENT

studno

d

undergraduate postgraduate

thesis title

year

STAFF

tutor

1

m

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Superclasses, SubclassesSpecialisation & Generalisation Relationships

ACADEMIC ADMIN

STAFF

TECHNICAL

payroll no name

length of service

levelgrade

project

O

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Superclasses, SubclassesSpecialisation & Generalisation Relationships

EMPLOYEE STUDENT

PERSON

O

name

salaryfee

O

d

RESEARCH TEACHINGPOSTGRAD

UNDERGRAD

thesis

O

LECTURING TUTORS

address

project

FINAL YEAR

year = 3

SUPERVISOR

1-2

1-2

courseno

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Categories and Categorisation a single superclass/subclass relationship with more

than one superclass, where the superclasses represent different entity types (sometimes with different keys)

PERSONCOMPANY

U

OWNER

personid compid

duration of ownership

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Specialisation & Generalisation Option A

1. Create a relation for superclass

2. Create a relation for each subclass such that:

{primary_key of superclass} U {attributes of subclass}

key for subclass is (primary_key of superclass)

Inclusion dependency:<key>(superclass) <key>(subclass )

Covering dependency:n (number of subclasses)

<key>(subclass ) = <key>(superclass)i=1

Disjoint dependency:n (number of subclasses)

<key>(subclass ) = i=1

name

given family

STUDENT

studno

d

undergraduate postgraduate

thesis title

year

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Specialisation & Generalisation Option B1. Create a relation for each subclass such that:

{primary_key U {attributes U {attributes of of superclass} of superclass} subclass}

key for each relation is (primary_key of superclass)

•Works for total and disjoint constraints•Partial: lose any entity that is not in a subclass•Overlapping: redundancy•To recover the superclass can do an OUTER UNION on the subclass relations

name

given family

STUDENT

studno

d

undergraduate postgraduate

thesis title

year

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Specialisation & Generalisation Option C1. Create one relation such that:

{primary_key U {attributes U {attributes U {type

of superclass} of superclass} of all subclasses} attribute} key for subclass is (primary_key of superclass)

•Many ‘not-applicable’ nulls•Does away with joins•Disjoint: one type which indicates which subclass the tuple represents•Overlap: set of types = number of subclasses•Partial: type is null represents superclass

name

given family

STUDENT

studno

d

undergraduate postgraduate

thesis title

year

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Specialisation & Generalisation Overlapping

1.STAFF(payrollno,name,lengthofservice)

ACADEMIC(payrollno,level)

TECHNICAL(payrollno,project)

ADMIN(payrollno,grade)

2.ACADEMIC(payrollno,name,lengthofservice,

level)

TECHNICAL(payrollno,name,lengthofservice,

project)

ADMIN(payrollno,name,lengthofservice,grade)

3.STAFF(payrollno,name,lengthofservice,level,

project,grade,type1,type2,type3)

STAFF(payrollno,name,lengthofservice,level,

project,grade,type)

type = powerset of classes

ACADEMIC ADMIN

STAFF

TECHNICAL

payroll no name

length of service

levelgrade

project

O

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Specialisation & Generalisation Relationships

EMPLOYEE STUDENT

PERSON

O

name

salaryfee

O

d

RESEARCH TEACHINGPOSTGRAD

UNDERGRAD

thesis

O

LECTURING TUTORS

address

project

FINAL YEAR

year = 3

SUPERVISOR

1-2

1-2

courseno

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Specialisation Lattice with Shared Subclass

To be a shared subclass the superclasses must have the same key.

Staffpayroll no

Academic AdminTechnical

d

d

Manager

Admin Manager

Hourly Staff

Salaried Staff

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Categories and Categorisation A category is a subclass of the union of two or more superclasses

that can have different keys because they can be of different entity types

If defining superclasses have different keys, specify a new surrogate key

PERSONCOMPANY

U

OWNER

personid compid

duration of ownership OWNER( ??? )PERSON( ??? )COMPANY( ??? )

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a)Mapping of EER to Relational model

Create a relation for the super class. with attributes {k,a1..an}

Create a relation Li for each Si

Attributes of Li={k}U{Attributes of Si}

Constraints can be disjoint/overlapping, Total/Partial

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b)Mapping of EER to Relational model

Create a relation Li for each Si

Attributes of Li={k a1..an}U{Attributes of Si}

Constraint must be disjoint and total

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c)Mapping of EER to Relational model

Create a single relation L Attributes of L={k a1..an}U{Attributes of S1}U…

U{Attributes of Sm}U{t} ‘t’ is a type attribute that indicates the sub-

class to which each tuple belongs. This may create a lot of null values

Constraint is disjoint (total or partial) Not very space efficient hence can use a

previous method But no need of ’joins’ to get all attributes

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d)Mapping of EER to Relational model

Create a single relation L Attributes of L={k a1..an}U{Attributes of S1}U…

U{Attributes of Sm}U{t1,t2 ,..tm } ‘t’ is a type attribute that indicates the sub-

class to which each tuple belongs. This may create a lot of null values

Constraint is overlapping (total or partial) Not very space efficient hence can use a

previous method But no need of ’joins’ to get all attributes

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Mapping shared sub-classes

Option ‘a’ is usually used Options ‘c’ & ‘d’ can also be used

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Mapping of categories

If the super-classes have different keys we have to define a new key attribute called a ‘surrogate key’. When creating a relation to correspond to the category.

Create a relation to correspond to the category,include any atributes of the category.The primary key is the surrogate key

Each super-class is also mapped into a relation with its own primary key, the surrogate key becomes a foreign key for this.

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Eg. For mapping category

Person(PersonID,name,…,OwnerID) Company(CompID,name,…,OwnerID) Owner(OwnerID, OwnerType)

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Hints for EER Modelling identify entity types by searching for nouns and noun phrases assume all entities are strong and check for weak ones on a later

pass need an identifier for each strong entity assume all relationships are partial participation (optional) and

check for total (mandatory) ones on a later pass expect to keep changing your mind about whether things are

entities, relationships or attributes keep level of detail relevant and consistent (for example leave out

attributes at first) approach diagram through different views and merge them

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Review: ISA Hierarchies

Contract_Emps

namessn

Employees

lot

hourly_wages

ISA

Hourly_Emps

contractid

hours_worked

As in C++, or other PLs, attributes are inherited. If we declare A ISA B, every A entity is also considered to be a B entity.

Overlap constraints: Can Joe be an Hourly_Emps as well as a Contract_Emps entity? (Allowed/disallowed)

Covering constraints: Does every Employees entity also have to be an Hourly_Emps or a Contract_Emps entity? (Yes/no)

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Translating ISA Hierarchies to Relations

General approach: 3 relations: Employees, Hourly_Emps and Contract_Emps.

• Hourly_Emps: Every employee is recorded in Employees. For hourly emps, extra info recorded in Hourly_Emps (hourly_wages, hours_worked, ssn); must delete Hourly_Emps tuple if referenced Employees tuple is deleted).

• Queries involving all employees easy, those involving just Hourly_Emps require a join to get some attributes.

Alternative: Just Hourly_Emps and Contract_Emps. Hourly_Emps: ssn, name, lot, hourly_wages,

hours_worked. Each employee must be in one of these two subclasses.

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Review: Binary vs. Ternary Relationships

What are the additional constraints in the 2nd diagram?

agepname

DependentsCovers

name

Employees

ssn lot

Policies

policyid cost

Beneficiary

agepname

Dependents

policyid cost

Policies

Purchaser

name

Employees

ssn lot

Bad design

Better design

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Binary vs. Ternary Relationships (Contd.)

The key constraints allow us to combine Purchaser with Policies and Beneficiary with Dependents.

Participation constraints lead to NOT NULL constraints.

CREATE TABLE Policies ( policyid INTEGER, cost REAL, ssn CHAR(11) NOT NULL, PRIMARY KEY (policyid). FOREIGN KEY (ssn) REFERENCES Employees, ON DELETE CASCADE)

CREATE TABLE Dependents ( pname CHAR(20), age INTEGER, policyid INTEGER, PRIMARY KEY (pname, policyid). FOREIGN KEY (policyid) REFERENCES Policies, ON DELETE CASCADE)

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Views

A view is just a relation, but we store a definition, rather than a set of tuples.

CREATE VIEW YoungActiveStudents (name, grade)AS SELECT S.name, E.gradeFROM Students S, Enrolled EWHERE S.sid = E.sid and S.age<21

Views can be dropped using the DROP VIEW

command. How to handle DROP TABLE if there’s a view on the

table?• DROP TABLE command has options to let the user

specify this.

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Views and Security

Views can be used to present necessary information (or a summary), while hiding details in underlying relation(s). Given YoungStudents, but not Students or

Enrolled, we can find students s who are enrolled, but not the cid’s of the courses they are enrolled in.

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Relational Model: Summary

A tabular representation of data. Simple and intuitive, currently the most widely

used. Integrity constraints can be specified by the

DBA, based on application semantics. DBMS checks for violations. Two important ICs: primary and foreign keys In addition, we always have domain constraints.

Powerful and natural query languages exist. Rules to translate ER to relational model

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Exercise: Relationships in ER Model

Define Works_In2 table that captures all the available information in the ER diagram.

lot

dname

budgetdid

sincename

Works_In2 DepartmentsEmployees

ssn

capacity

Locations

addr

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Review: Key Constraints

Each dept has at most one manager, according to the key constraint on Manages.

Translation to relational model?

Many-to-Many1-to-1 1-to Many Many-to-1

dname

budgetdid

since

lot

name

ssn

ManagesEmployees Departments

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Translating ER Diagrams with Key Constraints

Map relationship to a table: Note that did is

the key now! Why not ssn?

Separate tables for Employees and Departments.

Since each department has a unique manager, we could instead combine Manages and Departments.

CREATE TABLE Manages( ssn CHAR(11), did INTEGER, since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees, FOREIGN KEY (did) REFERENCES Departments)

CREATE TABLE Dept_Mgr( did INTEGER, dname CHAR(20), budget REAL, ssn CHAR(11), since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees)

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Review: Participation Constraints

Does every department have a manager? If so, this is a participation constraint: the

participation of Departments in Manages is said to be total (vs. partial).• Every did value in Departments table must

appear in a row of the Manages table (with a non-null ssn value!)lot

name dnamebudgetdid

sincename dname

budgetdid

since

Manages

since

DepartmentsEmployees

ssn

Works_In

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Participation Constraints in SQL

We can capture participation constraints involving one entity set in a binary relationship, but little else (without resorting to CHECK constraints).

CREATE TABLE Dept_Mgr( did INTEGER, dname CHAR(20), budget REAL, ssn CHAR(11) NOT NULL, since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees, ON DELETE NO ACTION)

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Review: Weak Entities

A weak entity can be identified uniquely only by considering the primary key of another (owner) entity. Owner entity set and weak entity set must

participate in a one-to-many relationship set (1 owner, many weak entities).

Weak entity set must have total participation in this identifying relationship set.

lot

name

agepname

DependentsEmployees

ssn

Policy

cost

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Translating Weak Entity Sets Weak entity set and identifying

relationship set are translated into a single table. When the owner entity is deleted, all owned

weak entities must also be deleted.CREATE TABLE Dep_Policy ( pname CHAR(20), age INTEGER, cost REAL, ssn CHAR(11) NOT NULL, PRIMARY KEY (pname, ssn), FOREIGN KEY (ssn) REFERENCES Employees, ON DELETE CASCADE)