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A Demo of Logical Database Design

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Aim of the demo

• To develop an understanding of the logical view of data and the importance of the relational model

• To consider entities, attributes, and tables within the relational view

• Learn basic and advanced ER modelling techniques

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Entities

• Relational database design starts by defining the required entities

• Entities are anything you want to store data about– people (students, customers, employees, etc.)– places (resorts, cities, countries, etc.)– things (restaurants, products, invoices, movies,

paintings, books, buildings, contracts, etc.)– events (elections, presentations, earthquakes, hurricanes,

etc.)

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• Entity sets are collections of related entities. Entities are related by their classification:

• student entities are related by the fact that they are all students

• invoice entities are related by the fact that they are all invoices

• car entities are related by the fact that they are all cars

Entity Sets / Entity Types

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• Unfortunately, database designers almost always use the two terms as synonyms.

• When database designers refer to entities, they really are referring to entity sets or entity types.

Therefore, when you see a reference to an EMPLOYEE “entity” in a database design, remember that EMPLOYEE actually represents an entity set/type that contains a collection of employee entities and each instance of this type is one single entity.

Entity Set (type) = Class

Entity = Object

Entity Sets/Types and Entities

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Attributes

• Properties of entities or relationships

• This is the data stored about an entity E.g. student entity could have attributes such as name,

address, DOB etc.

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Types of attributes• Simple – each entity has a single atomic value for the attribute. Cannot be

subdivided– Age, sex, marital status

• Composite (composed of several components): Can be subdivided into additional attributes– Address into street, city, zip– Name(FirstName, MiddleName, LastName)

• Single-valued: Can have only a single value– Person has one social security number

• Multi-valued: an entity may have multiple values for that attribute– Person may have several college degrees (qualifications, skills)– Color of a car

• Derived: Can be derived with algorithm– Age can be derived from date of birth

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Conceptual design: (ER Model is used at this stage.)

– What are the entities and relationships in the database?

– What information about these entities and relationships should we store in the database?

– What are the integrity constraints or business rules that hold?

– A database `schema’ in the ER Model can be represented pictorially (ER diagrams).

– Can map an ER diagram into a relational schema by hands or tools.

Overview of Database Design

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ER Diagram: Two Most Commonly Used Notations

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ER Diagrams - The Basics

Entity Type symbol Relationship

symbol

Primary Key

AttributesRelationship

name

Entity Type name

CourseNoCrsDescCrsUnits

Course

OfferNoOffLocationOffTime

Offering

Has

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ER Diagram Basics - Cardinalities

Inside symbol:minimum cardinality

CourseNoCrsDescCrsUnits

Course

OfferNoOffLocationOffTime

Offering

Has

Perpendicular line:one cardinality

Outside symbol:maximum cardinality

Circle: zerocardinality

Crow's foot: manycardinality

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Classification of Cardinalities

• Minimum cardinality - relationship participation– Mandatory: (existence dependent) Entity occurrence requires

corresponding occurrence in the related entity. Offering cannot exist without being related to a course.

– Optional: Entity occurrence does not require a corresponding occurrence in the related entity. A course does not necessarily need an offering to exist.

• Maximum cardinality based– 1-M– M-N– 1-1

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Weak Entities – Identifying Relationship• Identifying relationship

– One entity is existence-dependent on another

– PK of related entity contains PK component of parent entity

BldgIDBldgNameBldgLocation

Building

RoomNoRoomCapacity

Room

Contains

Identification Dependency Symbols: Solid relationship line for identifying

relationships Diagonal lines in the corners denote

weak entities.

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– A weak entity is an entity that• Is existence-dependent i.e. weak entities cannot exist without the owner

entity AND

• Has a primary key that is partially or totally derived from the parent entity in the relationship. They do not have the PK of their own

– However not every existence dependency results in a weak entity type.

Eg – Driver License cannot exist unless it is related to PERSON entity even though it has its own key (Licence number) and hence is not a weak entity.

Weak EntitiesWeak Entities

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M-N Relationships with Attributes

StdSSNStdName

StudentOfferNoOffLocationOffTime

Offering

EnrollsIn

EnrGrade

attribute of relationship

• Replace M-N relationship with

• Associative entity type

• Two identifying 1-M relationships

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Associative Entity Type Example

StdSSNStdName

StudentOfferNoOffLocationOffTime

Offering

EnrGrade

EnrollmentRegisters Grants

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– A recursive entity is one in which a relationship can exist between occurrences of the same entity set.

– A recursive entity is found within a unary relationship.

Recursive EntitiesRecursive Entities

FacSSNFacName

Faculty

a) manager-subordinate

Supervises

b) course prerequisites

CourseNoCrsDesc

Course PrereqTo

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Entity Supertypes and Subtypes

• Generalization hierarchy – Depicts relationships between higher-level supertype and

lower-level subtype entities

– Supertype has shared attributes

– Subtypes have unique attributes

– A subtype entity inherits its attributes and its relationships

from the supertype entity

– The supertype and its subtype(s) maintain an is-a relationship

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Generalization Hierarchies

EmployeeEmpNo

EmpNameEmpHireDate

...

SalaryEmpEmpSalary

HourlyEmpEmpRate

generalization hierarchy symbol

supertype

subtypes

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Disjointness and Completeness of Subtypes

• Disjointness: D means intersection is empty• Overlap: No symbol means intersection is not empty:

Faculty and Student. Subtypes have common entities

• Completeness: C means union of subtype entities is the set of supertype entities

• Nothing: supertype can have free standing entities ((not in any subtype). Union does not provide supertype

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Entity Vs Attribute• Should address be an attribute of Employees or an entity

(connected to Employees by a relationship)?• Depends upon the use we want to make of address

information, and the semantics of the data:• If we have several addresses per employee, address must be

an entity (since attributes cannot be set-valued).

• If the structure (city, street, etc.) is important, e.g., we want to retrieve employees in a given city, address must be modelled as an entity (since attribute values are atomic).

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Summary

• A data model is the relatively simple representation, usually graphic, of complex real-world data structures. It represents data structures and their characteristics, relations, constraints, and transformations.

• Different type attributes have different implications in a database. Some of these need to be considered carefully.

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Comprehensive Example

OfferingOfferNoOffLocationOffTime

EnrGrade

EnrollmentRegisters

Grants

CourseNoCrsDescCrsUnits

Course

FacultyFacSalaryFacRankFacHireHate

SSNNameCityStateZip

UnivPerson

Has

Teaches

Supervises

CStudentStdClassStdMajorStdGPA