Dependencies In DBMS

Before discussing about the dependencies let us know what is meant by Entity and Attribute.

Entity: It is a person, place, event or thing about which the information is mentioned. For example employee, customer, student are called entities.

Attribute:The characteristic property that describes an entity is called attribute. For example emp number of employee or customer id of customer entity.

Three type of dependencies in DBMS

1. Functional Dependency
2. Partial Functional Dependency
3. Transitive Dependency.

1. Functional Dependency:

• In a relation R, an attribute B said to be a functional dependent on attribute 'A' if the value of an attribute 'B' is uniquely determined by the value in the attribute 'A'
• The functional dependency 'B' on 'A' is represented as A-->B. B dependent on A.

2. Partial Functional Dependency:

• It is functional dependency in which some non key attributes will depends on part of key columns.

Ex:

Partial Functional Dependency

• In the above table name, sal depends on only empno
• But course completed date depends on empno, course title.
• To identify the row uniquely in the table empno, course title should be declared as key columns.
• Now non key attributes name, sal depends only on part of a key column, i.e, empno
• This type of functional dependency called as partial functional dependency.

3. Transitive Dependency:

• The functional dependency between two non key attributes is called           transitive dependency.

Ex: customer table

Transitive Dependency

• custid is identified as key column in customer table but the region dependent on only salesman.
• The functional dependency between non key attributes region, salesman is called the transitive dependency.

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