More Related Content
Similar to Rdbms xp 02 (20)
Rdbms xp 02
- 1. Database Design
Objectives
In this lesson, you will learn to:
Map an entity-relationship diagram to tables in relation to the
following:
Regular entities
Attributes
Relationships
Weak entities
Subtypes and supertypes
©NIIT RDBMS/Lesson 2/Slide 1 of 19
- 2. Database Design
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
©NIIT RDBMS/Lesson 2/Slide 2 of 19
- 3. Database Design
Mapping Entity-Relationship Diagrams to Tables
A database that conforms to an E/R diagram can be
represented by a collection of tables in the relational system
The mapping of E/R diagrams to tables can be discussed in
relation to the following:
Regular entities
Attributes
Relationships
Weak entities
Subtypes and Supertypes
©NIIT RDBMS/Lesson 2/Slide 3 of 19
- 4. Database Design
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
©NIIT RDBMS/Lesson 2/Slide 4 of 19
- 5. Database Design
Attributes
Each property or attribute shown in the E/R diagram
maps to a column in the appropriate table
©NIIT RDBMS/Lesson 2/Slide 5 of 19
- 6. Database Design
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
©NIIT RDBMS/Lesson 2/Slide 6 of 19
- 8. Database Design
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
©NIIT RDBMS/Lesson 2/Slide 8 of 19
- 9. Database Design
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
©NIIT RDBMS/Lesson 2/Slide 9 of 19
- 10. Database Design
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
©NIIT RDBMS/Lesson 2/Slide 10 of 19
- 11. Database Design
Tips on Logical Database Design (Contd.)
Keys:
Relational systems require keys that uniquely identify the
rows of a table.
There are various types of keys:
® Primary
® Foreign
® Candidate
® Alternate
® Composite
©NIIT RDBMS/Lesson 2/Slide 11 of 19
- 12. Database Design
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
®Acomposite key, on the other hand, comprises two or
more attributes
©NIIT RDBMS/Lesson 2/Slide 12 of 19
- 13. Database Design
Just a Minute…
Define the following terms:
a. Candidate Key
b. Alternate Key
©NIIT RDBMS/Lesson 2/Slide 13 of 19
- 14. Database Design
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.
©NIIT RDBMS/Lesson 2/Slide 14 of 19
- 16. Database Design
Summary
In this lesson, 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.
©NIIT RDBMS/Lesson 2/Slide 16 of 19
- 17. Database Design
Summary (Contd.)
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.
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.
©NIIT RDBMS/Lesson 2/Slide 17 of 19
- 18. Database Design
Summary (Contd.)
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.
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.
©NIIT RDBMS/Lesson 2/Slide 18 of 19
- 19. Database Design
Summary (Contd.)
To simplify multiple references, a new superentity should be
introduced; this is also called generalization.
©NIIT RDBMS/Lesson 2/Slide 19 of 19
