Introduction to Database System
Ghalib University
Computer Science Department
Zabihullah Rahmani
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Table of contents
Attribute and its different types
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Attributes
The properties or characteristics of an entity type in which the organization
interest to keep information is called attribute.
Each entity type has a set of attributes associated with it.
Example
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Cont..
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Types of Attributes
Attributes may be of different types.They may be:
Required Versus Optional Attributes
SimpleVersus Composite Attributes
Single valued Versus multi-valued Attributes
Stored Versus Derived Attributes
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RequiredVersus Optional Attributes
Required Attributes
An attribute that must be present for each entity instance is called a required
attribute.
E.g. Reg_No, Name
Optional Attribute
An attribute that may not have a value for every entity instance.
E.g. Overtime_Houre, Email
In various E-R diagramming notations, a symbol might appear in front
of each attribute to indicate whether it is required (e.g., *) or optional
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SimpleVersus Composite Attributes
Simple
A simple (or atomic) attribute is an attribute that cannot be broken down into
smaller components that are meaningful for the organization.
E.g. all the attributes associated with AUTOMOBILE are simple: Vehicle ID,
Color,Weight, and Horsepower.
Composite
An attribute that has meaningful component parts
E.g. Address which can be broken down into components like House_NO, Street,
City, State.
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Single valued Versus multi-valued Attributes
Single-Value Attributes
An Attribute that can take one and only one value for a given entity instance. e.g.
Reg_No, Roll_No
Multi-Value Attributes
An Attribute that may take more than one value for a given entity instance. e.g.
Phone_No, Skill.
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Store versus Derived Attributes
Store Attributes
An attributes that value are store in database are called store attributes. E.g.
Reg_NO, name, address, phone, etc.
Derived Attributes
An attribute whose value can be calculated from related attribute values is called
derived attributes. E.g. EMP_AGE.
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Symbols For Attributes
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Example
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Identifier Attribute
The ERM uses identifiers, that is, one or more attributes that uniquely
identify each entity instance.
No two instances of the entity type may have the same value for the identifier
attribute.
In the relational model, such identifiers are mapped to primary keys (PKs) in
tables. Identifiers are underlined in the ERD.
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Choosing Identifier Rules
Choose an identifier that will not change its value over the life of each
instance of the entity type.
Choose an identifier such that for each instance of the entity, the attribute
is guaranteed to have valid values and not be null (or unknown).
Try to select single attribute for identifier.
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Naming and Defining Attributes
The general guidelines for naming:
An attribute name is a singular noun. Such as Customer_ID, Age
An attribute name should be unique
An attribute definition should make it clear what is included and not
included in the attribute’s value
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Any question…?????
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