02 rdbms session02

Relational Database Management Systems
Objectives

In this session, you will learn to:
Map an ER diagram to a table

Ver. 1.0 Session 2 Slide 1 of 16

Conceptual Model

Reflects entities and their relationships, based on the data
processing needs of an organization
Can be mapped to a relational, hierarchical, or network
model
Is independent of individual applications, database
management systems, hardware, and physical storage of
data


Mapping ERDs to Tables

• A database that conforms to an ER diagram can be
represented by a collection of tables in the relational
system.
• The mapping of ER diagrams to tables can be discussed in
relation to the following:
Regular entities
Attributes
Relationships
Weak entities
Subtypes and Supertypes


Regular Entities

They can exist in isolation, independent of any other entity.
They are the “building blocks” of the database.
Each regular entity maps to a table.


Attributes

Each property or attribute shown in the ER diagram maps to
a column in the appropriate table.


Relationships

Each type of relationship maps to tables in a different
manner in the relational database management system.
The most important principle is to create tables where
information from the real world is stored and retrieved in an
optimal way; that is, a minimum number of tables with a
minimum number of attributes.
In a relational system, a join operation retrieves all
information by combining two or more tables.
Are of the following types:
One-to-One
One-to-Many
Many-to-Many


Weak Entities

A weak entity is an entity whose existence depends on
some other entity.
It cannot exist if the entity on which it depends does not
exist.


Subtypes and Supertypes

Each entity type (subtypes and supertypes) maps to a
separate table.
The primary key of the supertype is the foreign key of the
subtype:
It creates a link between the two.
The foreign key of the subtype is also its primary key.


Tips on Logical Database Design

Attributes:
Do not introduce any unnecessary attributes.
An attribute serves three purposes:
To identify its owner entity
To refer to another entity
To simplify the description of an entity
Keys:
Relational systems require keys that uniquely identify the rows
of a table.
There are various types of keys:
Primary
Foreign
Candidate
Alternate
Composite


Tips on Logical Database Design (Contd.)

Any attribute (or set of attributes) that uniquely identifies a row
in a table is a candidate for the primary key.
Such an attribute is called a candidate key.
Any attribute that is a candidate for the primary key but is not
the primary key is called the alternate key.
Keys can be simple or composite:
A simple key is composed of a single attribute.
A composite key, on the other hand, comprises two or more
attributes.


Just a minute

Define the following terms:
1. Candidate Key
2. Alternate Key

Answer:
1. Any attribute (or set of attributes) that uniquely identifies a row
in a table is a candidate for the primary key. Such an attribute
is called a candidate key.
2. Any attribute that is a candidate for the primary key but is not
the primary key is called the alternate key.


Tips on Logical Database Design (Contd.)

Entities:
Some attributes may acquire further attributes during database
design and become entities.
Subentities:
Replace optional attributes of an entity with subentities. This is
called specialization.
Specialization is the result of taking a subset of a higher-level
entity set to form a lower-level entity set.
Generalization is the result of taking the union of two or more
lower-level entity sets to produce a higher‑level entity set.


Demo: Mapping the ER Diagram to Tables

Problem Statement
Map the ER diagram to its corresponding tables.


Summary

In this session, you learned that:
The conceptual model reflects entities and their relationships.
Data analysis helps determine entities and relationships. The
conceptual model is independent of the system where it is to
be implemented.
Regular entities are not dependent. They can exist in isolation,
independent of any other entity.
Each entity maps to a table. Each attribute in an E/R diagram
maps to an attribute in a table.
Entities with common attributes should be merged. Attributes
may acquire further attributes and become entities.
The mapping of relationships depends on the type of
relationship. Each type of relationship maps to tables in a
different manner in the relational database management
system.


Summary (Contd.)

In one-to-one relationships, one instance of an entity can relate
to only one instance of the related entity.
In a one-to-many relationship, one instance of an entity can
relate to more than one instance of the related entity.
Many-to-many relationships map to tables. One-to-one
relationships are not very common and may map to foreign
keys in tables.
A weak entity is an entity whose existence depends on some
other entity.
A subtype is a subset of another entity. A subtype is always
dependent on supertype for its existence.
The primary key of the supertype is the foreign key of the
subtype. It creates a link between the two.


Summary (Contd.)

A candidate key is a candidate for the primary key. An
alternate key is a candidate key that is not a primary key.
Optional attributes should be replaced with subentities; this is
also called specialization.
To simplify multiple references, a new superentity should be
introduced; this is also called generalization.


02 rdbms session02

Recommandé

Recommandé

Contenu connexe

Tendances

Tendances (13)

Similaire à 02 rdbms session02

Similaire à 02 rdbms session02 (20)

Plus de Niit Care

Plus de Niit Care (20)

Dernier

Dernier (20)

02 rdbms session02

Notes de l'éditeur