CSCI 242Relational Data Modeling

Copyright 2011, David C. Roberts, all rights reserved

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Relational Database

Remember that a relational database is a collection of time-varying, independent relations

Each relation models some entity type in the problem space (outside the computer)

There is a close correspondence between entity types and relations

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Data Modeling

• We can say that the relational database models the real-world problem

• So construction of a relational database becomes the selection of the entity types to put into the data model

• Sometimes entity type selection is simple; other times entity types are not apparent and selection can be difficult

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Notation for Data Models

• Chen developed the entity-relation data model, which considered entity types and their relationships

• Originally he expected that a new type of database system would evolve, directly implementing the E-R data model

• Instead, the E-R notation was found to be an excellent tool for relational database design and the relational approach has taken over

• Since then, popular automated tools have tended to change the notation for easier computer printing

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Chen’s Notation

• We will use Chen’s notation today for two reasons:

1. It separates the notion of a relationship from the notion of an entity type

2. Conversion of the design into relational tables occurs at the end of the design process, so the difference between the data model and physical design is clear

• In your own work, once you are familiar with the concepts, you will be able to use the popular “crow’s foot” notation used by DBDesigner4

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Entities

• Strong Entity—has independent existence

• Weak Entity—exists only when the entity it depends on exists

Employee

Dependent

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Examples of Entity Types

EMPLOYEECUSTOMERORGANIZATIONPARTINGREDIENTPURCHASE ORDERCUSTOMER ORDERPRODUCT

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Examples of Entity Instances

An instance of an entity is a specific occurrence of an entity type:

• Bill Gates is an Employee of Microsoft• Spam is a Product• Greenpeace is an Organization• Flour is an ingredient

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Attributes

• Attributes are indicated inside ovals; identifier attributes are underlined. For complex diagrams, attributes are often omitted

Employee

EMPNO

JOB

SALARY

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Examples of Attributes

An attribute is a characteristic of an entity type:EmployeeIDSocial Security NumberFirst NameLast NameStreet AddressCityStateZipCodeDate Hired Health Benefits Plan

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MVD Attribute

• An MVD attribute is shown as a double line

Employee

EMPNO

JOB

SALARY

TELEPHONE

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Relationship

A relationship is an association between two entity types, for example:– A CUSTOMER places a CUSTOMER ORDER

– An EMPLOYEE takes a CUSTOMER ORDER– A STUDENT enrolls in a COURSE– A COURSE is taught by a FACULTY

MEMBER• Some say that entity types should be nouns and

relationships should be verbs

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Relationship

EMPLOYEE

DEPARTMENT

WORKS FOR

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Categorizing Relationships

• Number of entity types participating: – Unary: one– Binary: two– Ternary: three

• Existence of related instances(a.k.a. optionality):– Mandatory– Optional

• Cardinality of the relationship:– One-many– Many-many– One-one

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One-to-One Relationships

• Usually the only time a one-to-one relationship is used is for a dependent entity. Otherwise, usually if there is a one-to-one relationship, careful consideration will show that there is just one entity type.

EMPLOYEE

SPOUSE

HAS

1

1

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Relationship And Cardinality

• A relationship is an association between two or more entity types, drawn as a diamond. Relationships may be one-to-many, many-to-many or one-to-one

DEPARTMENT

EMPLOYEE

HAS

STUDENT

COURSE

GRADE

EMPLOYEE

COMPANY CAR

HAS

1

N N

N 1

1

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Recursive Relationship

• Recursive relationship is how a repeated hierarchy is represented

Employee WORKS FOR

Question: How else can this hierarchy be represented?

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Relationship of Higher Degree

• A ternary relationship, also said to have degree 3.

STUDENT

COURSE

REG

N

N

TEACHERN

COURSENO

GRADE

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Optionality of Participation

EMPLOYEE

HAS

STUDENT

COURSE

GRADE

EMPLOYEE

COMPANY CAR

HAS

1

N N

N 1

1

DEPARTMENT

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Subtypes

• It can be useful to consider a supertype that includes several entity types as subtypes

• Supertypes can be– Complete: every instance of the supertype is

one of the subtypes– Distinct: no single entity type can be a

member of two subtypes

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Notation

• Chen’s notation for supertypes and subtypes is cumbersome, so let’s use “crow’s-foot” notation

• A supertype is shown as a box surrounding the boxes for all the subtypes

• Relationship lines end on the supertype or the subtype boundaries as appropriate

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Subtypes

• Subtypes can be either mutually exclusive (disjoint) or overlapping (inclusive). – For a mutually exclusive category, an entity

instance may be in only one subtype. – For an overlapping category, an entity

instance may be in two or more subtypes. • The completeness constraint: all

instances of a subtype must be represented in the supertype.

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Example SubtypesPARTY

EMPLOYEE

APPLICANT

PERSON

ORGANIZATION

SUPPLIER

CUSTOMER

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Why Use Subtypes and Supertypes Like This?

• Relationships are simplified– The relationship can take place at the highest

possible level in the hierarchy– Fewer relationship tables are generally

required• Programming can be simplified

– All subtypes can be processed in the same way

– For example, write a check to an organization or a check to a supplier with the same code

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Converting an ERD to Relational

• Recall that when we did an ERD we were not designing tables, we were defining the problem in terms of entity types and relationships

• Now the design must be translated into relational tables

Question: What are the symbols on the ERD that will become tables in the relational database?

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Converting to Relational

• Every entity type becomes a table• For one-many relationships, put the

identifier of the one with each of the many• For one-one relationships, put the

identifier of each with the other• For many-many relationships, identify an

entity type that connects them, and make it a table, using the primary key of both partners as a composite primary key

Conversion

• We see that every rectangle will become a table in the relational database

• Some of the diamonds will become tables and some will not

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Students and Courses

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Course

NumberLocation

Credits

Registration

Grade

StudentIDName

Student

Grad YR

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What About Subtypes?

• Each subtype and supertype is a separate table

• Common attributes are stored with the highest level entity that shares them

• The root level supertype may have many attributes, or it may have only a key