# Functional Dependency

A functional dependency in databases is a dependency which satisfies the properties of a function.

A dependency exists in a database table when an attribute value is dependent on the value of a single or a group of attributes. The dependency is functional when it satisfies the properties of a function and the main property of a function is that it gives a single value for some input. In other words, if the value of dependent attribute/s is same for some value of the determinant attribute/s in all the tuples then it is a functional dependency.

## Symbol

A functional dependency is denoted by ‘→’

e.g. A → B
Where A is the determinant attribute and B is the dependent attribute. A → B can be read as A determines B.

## Types of functional dependencies

Trivial dependency: If the dependent is a subset of the determinant. As the name suggests trivial dependency is something which does exist but is of no importance.
e.g. AB → B

Non-Trivial dependency: In non-trivial dependency the dependent part is not a subset of the determinant part.

Transitive dependency: If a functional dependency is achieved through two functional dependencies indirectly then the dependency is called transitive dependency.
e.g. A → B and B → C suggests A → C

## Example

To find out if a dependency is functional check whether the dependent attribute is same for some value of the determinant attribute through out the table.

Consider the following table

Col1 Col2 Col3
A 3 5
B 5 4
A 3 8
B 5 4

Col1 → Col2 is valid since the value is same for both As and both Bs. Col1 → Col3 is invalid since for the second A the value of Col3 is 8 which should’ve been 5.

by Arifullah Jan and last modified on

XPLAIND.com is a free educational website; of students, by students, and for students. You are welcome to learn a range of topics from accounting, economics, finance and more. We hope you like the work that has been done, and if you have any suggestions, your feedback is highly valuable. Let's connect!