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
*

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
*

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
*

Properties of a Relation

*

Example Relation / Table

*

Attributes and Domains

*

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.
*

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
*

*

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
*

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
*

Keys (cont..)

*

Keys (cont..)

*

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
*

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
*

Integrity Rules

*

Integrity Rules

*

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
*

*

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
*

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
*

The 1:M Relationship

Relational database norm
Found in any database environment
*

*

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
*

*

The 1:1 Relationship (cont…)

*

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
*

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
*

The M:M Relationship (cont..)

*

The M:M Relationship (cont…)

*

Figure 3.16 in the book

*

Figure 3.17 in your book

*

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
*

Data Redundancy Revisited (cont…)

*

Data Redundancy Revisited (cont..)

*

*

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
*

*

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
*

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.

*