Functional Dependancy

Posted By on October 7, 2014

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Armstrong's Axioms


Functional Dependancy

A functional dependency is an association between two attributes of the same relational database table. One of the attributes is called the determinant and the other attribute is called the determined. For each value of the determinant there is associated one and only one value of the determined.

If A is the determinant and B is the determined then we say that A functionally determines B and graphically represent this as A -> B. The symbols A à B· can also be expressed as B is functionally determined by A.




Since for each value of A there is associated one and only one value of B.


A does not functionally determine B

Since for A = 3 there is associated more than one value of B.

Functional dependency can also be defined as follows:

An attribute in a relational model is said to be functionally dependent on another attribute in the table if it can take only one value for a given value of the attribute upon which it is functionally dependent.

Example: Consider the database having following tables:

Functional dependency

Here in Supplier table

Sno              –             Supplier number of supplier that is unique

Sname       –             Supplier name

City             –             City of the supplier

Status      –     Status of the city e.g. A grade cities may have status 10, B grad cities             may have status 20 and so on.

Here, Sname is FD on Sno. Because, Sname can take only one value for the given value of Sno (e.g. S 1) or in other words there must be one Sname for supplier number S1.

FD is represented as:

Sno à Sname

FD is shown by à which means that Sname is functionally dependent on Sno.

Similarly, city and status are also FD on Sno, because for each value of Sno there will be only one city and status.

FD is represented as:

Sno   – City

Sno   – Status

S. Sno – S (Sname, City, Status)


Consider another database of shipment with following attributes:

Sno     –     Supplier number of the supplier

Pno     –     Part number supplied by supplier

Qty     –     Quantity supplied by supplier for a particular Part no

In this case Qty is FD on combination of Sno, Pno because each combination of Sno and Pno results only for one Quantity.

SP (Sno, Pno) –> SP.QTY


Dependency Diagrams

A dependency diagram consists of the attribute names and all functional dependencies in a given table. The dependency diagram of Supplier table is.

Dependency diagrams

Here, following functional dependencies exist in supplier table

Sno         –       Sname

Sname     –       Sno

Sno         –       City

Sno         –       Status

Sname     –     City

Sname     –     Status

City          –     Status

The FD diagram of relation P is

FD diagram

Here following functional dependencies exist in Part table:


Pno – Pname

Pno – Color

Pno – Wt

functional dependencies exist

The FD diagram of relation Shipment is

Here following functional dependencies exist in parts table

SP (Sno, Pno) – SP.QTY


Armstrong's Axioms

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Posted by Akash Kurup

Founder and C.E.O, World4Engineers Educationist and Entrepreneur by passion. Orator and blogger by hobby