Rdbmss enforce integrity rules automatically.

DB-Lecture3_ch03.ppt

Database Principles: Fundamentals of Design, Implementations and Management

CHAPTER 3

Relational Model Characteristics

Objectives

In this chapter, you will learn:

That the relational database model offers a logical view of data

About the relational model’s basic component: relations

That relations are logical constructs composed of rows (tuples) and columns (attributes)

That relations are implemented as tables in a relational DBMS

About relational database operators, the data dictionary, and the system catalog

How data redundancy is handled in the relational database model

Why indexing is important

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A Logical View of Data

Relational model

Enables the programmer to view data logically rather than physically

Table

Has structural and data independence

Resembles a file conceptually

Relational database model easier to understand than its hierarchical and network database predecessors models

Table also called a relation because the relational model’s creator, Codd, used the term relation as a synonym for table

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Tables and Their Characteristics

Logical view of relational database based on relation

Relation thought of as a table

Think of a table as a persistent relation:

A relation whose contents can be permanently saved for future use

Table: two-dimensional structure composed of rows and columns

Persistent representation of logical relation

Contains group of related entities = an entity set

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Properties of a Relation

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Example Relation / Table

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Attributes and Domains

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Each attribute is a named column within the relational table and draws its values from a domain.

The domain of values for an attribute should contain only atomic values and any one value should not be divisible into components.

No attributes with more than one value are allowed.

Degree and Cardinality

Degree and cardinality are two important properties of the relational model.

A relation with N columns and N rows is said to be of degree N and cardinality N.

The degree of a relation is the number of its attributes and the cardinality of a relation is the number of its tuples.

The product of a relation’s degree and cardinality is the number of attribute values it contains.

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Relational Schema

A relational schema is a textual representation of the database tables, where each table is described by its name followed by the list of its attributes in parentheses.

Keys

A key consists of one or more attributes that determine other attributes

Primary key (PK) is an attribute (or a combination of attributes) that uniquely identifies any given entity (row)

A Key’s role is based on determination

If you know the value of attribute A, you can look up (determine) the value of attribute B

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Keys (cont..)

Relational Database Keys (cont….)

Composite key

Composed of more than one attribute

Key attribute

Any attribute that is part of a key

Superkey

Any key that uniquely identifies each row

Candidate key

A superkey without redundancies and without unnecessary attributes

Ex: Stud_ID, Stud_lastname

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Keys (cont..)

Nulls:

No data entry

Not permitted in primary key

Should be avoided in other attributes

Can represent

An unknown attribute value

A known, but missing, attribute value

A “not applicable” condition

Can create problems when functions such as COUNT, AVERAGE, and SUM are used

Can create logical problems when relational tables are linked

Controlled redundancy:

Makes the relational database work

Tables within the database share common attributes that enables the tables to be linked together

Multiple occurrences of values in a table are not redundant when they are required to make the relationship work

Redundancy exists only when there is unnecessary duplication of attribute values

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Keys (cont..)

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Keys (cont..)

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Keys (cont..)

Foreign key (FK)

An attribute whose values match primary key values in the related table

Referential integrity

FK contains a value that refers to an existing valid tuple (row) in another relation

Secondary key

Key used strictly for data retrieval purposes

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Integrity Rules

Many RDBMs enforce integrity rules automatically

It is safer to ensure that your application design conforms to entity and referential integrity rules

Rules are summarized in the next slide

Designers use flags to avoid nulls

Flags indicate absence of some value

For Ex, the code -99 could be used as the AGENT_CODE entry for the 4th row of the CUSTOMER Table to indicate that customer Paul Olowsky does not have yet an agent assigned to it

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Integrity Rules

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Integrity Rules

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The Data Dictionary and System Catalog

Data dictionary

Provides detailed accounting of all tables found within the user/designer-created database

Contains (at least) all the attribute names and characteristics for each table in the system

Contains metadata: data about data

Sometimes described as “the database designer’s database” because it records the design decisions about tables and their structures

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A Sample Data Dictionary

The Data Dictionary and System Catalog (cont..)

System catalog

Contains metadata

Detailed system data dictionary that describes all objects within the database

Terms “system catalog” and “data dictionary” are often used interchangeably

Can be queried just like any user/designer-created table

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Relationships within the Relational Database

1:M relationship

Relational modeling ideal

Should be the norm in any relational database design

1:1 relationship

Should be rare in any relational database design

M:N relationships

Cannot be implemented as such in the relational model

M:N relationships can be changed into two 1:M relationships

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The 1:M Relationship

Relational database norm

Found in any database environment

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The 1:M Relationship (cont…)

The 1:1 Relationship

One entity related to only one other entity, and vice versa

Sometimes means that entity components were not defined properly

Could indicate that two entities actually belong in the same table

Certain conditions absolutely require their use

As rare as 1:1 relationships should be, certain conditions absolutely require their use

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The 1:1 Relationship (cont…)

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The 1:1 Relationship (cont…)

The M:N Relationship

Can be implemented by breaking it up to produce a set of 1:M relationships

Avoid problems inherent to M:N relationship by creating a composite entity or a bridge entity

The composite entity Includes -as foreign keys- at least the primary keys of the tables that are to to be linked

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Implementation of a composite entity

Yields required M:M to 1:M conversion

Composite entity table must contain at least the primary keys of original tables

Linking table contains multiple occurrences of the foreign key values

Additional attributes may be assigned as needed

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The M:M Relationship (cont..)

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The M:M Relationship (cont…)

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Figure 3.16 in the book

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Figure 3.17 in your book

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Data Redundancy Revisited

Data redundancy leads to data anomalies

Such anomalies can destroy the effectiveness of the database

Foreign keys

Control data redundancies by using common attributes shared by tables

Crucial to exercising data redundancy control

Sometimes, data redundancy is necessary

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Data Redundancy Revisited (cont…)

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Data Redundancy Revisited (cont..)

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Data Redundancy Revisited (cont…)

Indexes

Orderly arrangement to logically access rows in a table

Index key

Index’s reference point

Points to data location identified by the key

Unique index

Index in which the index key can have only one pointer value (row) associated with it

Each index is associated with only one table

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Indexes (cont..)

Similar to Figure 3.20 of your book and better explained

Codd’s Relational Database Rules

In 1985, Codd published a list of 12 rules to define a relational database system

The reason was the concern that many vendors were marketing products as “relational” even though those products did not meet minimum relational standards

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Summary

Tables (relations) are basic building blocks of a relational database

Keys are central to the use of relational tables

Keys define functional dependencies

Superkey

Candidate key

Primary key

Secondary key

Foreign key

Each table row must have a primary key which uniquely identifies all attributes

Tables can be linked by common attributes. Thus, the primary key of one table can appear as the foreign key in another table to which it is linked

Good design begins by identifying appropriate entities and attributes and the relationships among the entities. Those relationships (1:1, 1:M, M:N) can be represented using ERDs.

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