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Types of Attributes
Attributes can be of various types. In this section, we'll look at different types
of attributes. Attributes can be categorized as:
Key or non-key attributes
Attributes can be classified as identifiers or descriptors. Identifiers, more
commonly called keys or key attributes uniquely identify an instance of an entity.
If such an attribute doesn't exist naturally, a new attribute is defined for that
purpose, for example an ID number or code. A descriptor describes a non-unique
characteristic of an entity instance.
An entity usually has an attribute whose values are distinct for each individual
entity. This attribute uniquely identifies the individual entity. Such an attribute
is called a key attribute. For example, in the Employee entity type, EmpNo is
the key attribute since no two employees can have same employee number. Similarly,
for Product entity type, ProdId is the key attribute.
There may be a case when one single attribute is not sufficient to identify
entities. Then a combination of attributes can solve this purpose. We can form
a group of more than one attribute and use this combination as a key attribute.
That is known as a composite key attribute. When identifying attributes of entities,
identifying key attribute is very important.
Required or optional Attributes
An attribute can be required or optional. When it's required, we must have
a value for it, a value must be known for each entity occurrence. When it's optional,
we could have a value for it, a value may be known for each entity occurrence.
For example, there is an attribute EmpNo (for employee no.) of entity employee.
This is required attribute since here would be no employee having no employee
no. Employee's spouse is optional attribute because an employee may or may not
have a spouse.
Simple and composite Attributes
Composite attributes can be divided into smaller subparts. These subparts represent
basic attributes with independent meanings of their own. For example, take Name
attributes. We can divide it into sub-parts like First_name, Middle_name, and
Last_name.
Attributes that can’t be divided into subparts are called Simple or Atomic
attributes. For example, EmployeeNumber is a simple attribute. Age of a person
is a simple attribute.
Composite Attributes
Single-valued and multi-valued
Attributes
Attributes that can have single value at a particular instance of time are
called singlevalued. A person can’t have more than one age value. Therefore,
age of a person is a single-values attribute. A multi-valued attribute can have
more than one value at one time. For example, degree of a person is a multi-valued
attribute since a person can have more than one degree. Where appropriate, upper
and lower bounds may be placed on the number of values in a multi-valued attribute.
For example, a bank may limit the number of addresses recorded for a single customer
to two.
Stored, coded, or derived Attributes
There may be a case when two or more attributes values are related. Take the
example of age. Age of a person can be can be calculated from person’s date
of birth and present date. Difference between the two gives the value of age.
In this case, age is the derived attribute.
The attribute from which another attribute value is derived is called stored
attribute. In the above example, date of birth is the stored attribute. Take another
example, if we have to calculate the interest on some principal amount for a given
time, and for a particular rate of interest, we can simply use the interest formula
Interest=NPR/100;
In this case, interest is the derived attribute whereas principal amount(P),
time(N) and rate of interest(R) are all stored attributes.
Derived attributes are usually created by a formula or by a summary operation
on other attributes.
A coded value uses one or more letters or numbers to represent a fact. For
example, the value Gender might use the letters "M" and "F"
as values rather than "Male" and "Female".
The attributes reflect the need for the information they provide. In the analysis
meeting, the participants should list as many attributes as possible. Later they
can weed out those that are not applicable to the application, or those clients
are not prepared to spend the resources on to collect and maintain. The participants
come to an agreement, on which attributes belong with an entity, as well as which
attributes are required or optional.
See Also
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