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Gegevens analyse

Les 1

Fedor Wagenaar

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Data in Tables

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The Key Characteristic of

Databases: Related Tables

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Database Examples

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MS Access in Detail

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Enterprise-Class Database

Systems

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The Database

A database is a self-describing collectionof integrated tables

The tables are called integrated becausethey store data about the relationshipsbetween the rows of data

A database is called self-describing

because it stores a description of itself The self-describing data are called

metadata, which is data about data

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Typical Metadata Tables

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Database Contents

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Three Types ofDatabase Design

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Database Design from Existing Data

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Data Import: One or Two Tables?

This is an important

decision, and based

on a set of rules

known as

normalization (whichis covered in Chapter

Three)

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Database Design from New

Systems

Development

Entity-Relationship data modeling is covered in Chapter Five, and data model

transformations to database designs are covered in ChapterSix

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Database Design from Database

Redesign

Database redesign is

covered in Chapter

Eight, after coverage of

SQL in ChapterSeven

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Knowledge Priorities

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The Data Model

A data model is a plan, or blueprint, for a

database design.

A data model is more generalized andabstract than a database design.

It is easier to change a data model than it

is to change a database design, so it is the

appropriate place to work through

conceptual database problems.

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Entities

Something that can be identified and theusers want to track

Entity class a collection of entities of a

given type Entity instance the occurence of a

particular entity

There are usually many instances of anentity in an entity class.

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CUSTOMER:The Entity Class and Two Entity Instances

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Attributes

Attributes describe an entityscharacteristics.

All entity instances of a given entity classhave the same attributes, but vary in thevalues of those attributes.

Originally shown in data models as

ellipses. Data modeling products today commonly

show attributes in rectangular form.

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EMPLOYEE:

Attributes in Ellipses

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EMPLOYEE:

Attributes in Entity Rectangle

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Identifiers

Identifiers are attributes that name, or identify, entity

instances.

The identifier of an entity instance consists of one or

more of the entitys attributes. Composite identifiers: Identifiers that consist of two or

more attributes

Identifiers in data models become keys in database

designs:

Entities have identifiers.

Tables (or relations) have keys.

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Entity Attribute Display

in Data Models

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Relationships

Entities can be associated with one another in

relationships: Relationship classes: associations among entity classes

Relationship instances: associations among entity instances

In the original E-R model, relationships could

have attributes but today this is no longer done.

A relationship class can involve two or more

entity classes.

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Degree of the Relationship

The degree of the relationship is the number of

entity classes in the relationship:

Two entities have a binary relationship of degree

two. Three entities have a ternary relationship of degree

three.

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Binary Relationship

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Ternary Relationship

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Entities and Tables

The principle difference between an entity

and a table (relation) is that you can

express a relationship between entities

without using foreign keys.

This makes it easier to work with entities in

the early design process where the very

existence of entities and the relationshipsbetween them is uncertain.

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Cardinality

Cardinality means count, and is

expressed as a number.

Maximum cardinality is the maximumnumber of entity instances that can

participate in a relationship.

Minimum cardinality is the minimum

number of entity instances that must

participate in a relationship.

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Maximum Cardinality

Maximum cardinality is the maximum

number of entity instances that can

participate in a relationship.

There are three types of maximum

cardinality:

One-to-One [1:1]

One-to-Many [1:N]

Many-to-Many [N:M]

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The Three Types of

Maximum Cardinality

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Parent and Child Entities

In a one-to-many relationship: The entity on the one side of the relationship is called

the parent entity or just the parent.

The entity on the many side of the relationship iscalled the child entity or just the child.

In the figure below, EMPLOYEE is the parentand COMPUTER is the child:

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Minimum Cardinality

Minimum cardinality is the minimum number ofentity instances that must participate in arelationship.

Minimums are generally stated as either zero orone: IF zero [0] THEN participation in the relationship by

the entity is optional, and no entity instance mustparticipate in the relationship.

IF one [1] THEN participation in the relationship bythe entity is mandatory, and at least one entityinstance must participate in the relationship.

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Indicating Minimum Cardinality

As shown in the examples in a following

slide:

Minimum cardinality of zero [0] indicating

optional participation is indicated by placing

an oval next to the optional entity.

Minimum cardinality of one [1] indicating

mandatory (required) participation isindicated by placing a vertical hash mark

next to the required entity.

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Reading Minimum Cardinality

Look toward the entity in question:

IF you see an oval THEN that entity is

optional (minimum cardinality of zero [0]).

IF you see a vertical hash mark THEN that

entity is mandatory (required) (minimum

cardinality of one [1]).

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The Three Types of

Minimum Cardinality

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Data Modeling Notation

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Data Modeling Notation:

ERwin

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Data Modeling Notation:

N:M and O-MNote that:

(1) ERwin cannotindicate trueminimum

cardinalities onN:M relationships

(2) Visio introducesthe intersectiontable instead of

using a true N:Mmodel