What is an attribute? What are the various types of attributes?

 

Attribute and its various types:

 

 Attribute:

 

An entity is represented by a set of attributes. Attributes are descriptive properties possessed by each member of an entity set. Each entity has a value for each of its attributes. For ex: a particular customer entity may have the value 12345 for Customer-ID.

 

For each attribute there is a set of permitted values called the domain, or value set of that attribute. The domain of attribute customer name might be the set of all text strings of a certain length.

 

Types of attributes:

 

1.    Simple and Composite Attribute:

 

If attributes cannot be further divided into sub-parts then they are known as simple attribute. Composite attributes on the other hand can be divided into sub parts. For ex: an attribute name would be structured as a composite attribute consisting of First-name, Middle-name, and Last-name.

Fig: Example of composite attributes

2.    Single and Multi-valued attributes:

 

The Loan-number attribute for a specific loan entity refers to only one loan number. Such attribute is called as single valued attribute. There may be instances where an attribute has a set of values for a specific entity. Consider an employee entity set with the attribute phone number. An employee may have zero, one or several phone numbers and different employees may have different numbers of phones. This type of attribute is said to be multi-valued attribute.

 

3.    Derived attributes:

 

The value for this type of attribute can be derived from the values of other related attributes or entities. As an example, suppose that the Customer entity set has an attribute age which indicates the customer’s age. If the customer entity set also has an attribute date-of-birth, we can calculate age from date of birth and the current date. So, age is a derived attribute. In this example, date-of-birth may be referred as a base attribute.

Fig: Example of composite, multi-valued and derived attributes

4.     Null attribute:

 

An attribute takes a null value when an entity does not have a value for it. Null value may indicate not applicable, i.e. the value does not exist for the entity. For ex: one may have no middle name. Null can also designate that an attribute value is unknown. An unknown value may be either missing or not known.