Tuesday, August 26, 2014

How to find key of a given table? - An alternate way

Closure of Attribute Set - Alternate Way / Closure of Attribute Set Algorithm / How do we find a key given a set of attributes? / How to find super key? / Attribute Closure in DBMS / Closure of Attributes Set Example

To find the Closure of attribute/attribute set A, we have to do the following;

          a) First of all find the set of all functional dependencies (F+) that are implied by a given set of functional dependencies (F).
            b) Then take all the FDs from F+ that has A on the left hand side of the arrow.
          C) Finally, find the union of right hand side of all those functional dependencies that have A on the left hand side. If the union includes all the attributes of the given relation R, then A is the key for the relation.

Assume a relation schema R = (A, B, C) with the set of functional dependencies F = {A B, B C}. Now, we can find the closure of attribute A as follows, according to this method;
Step 1: Find the F+.
F+ = { A B, B C, A C } [A C is derived using Transitivity rule]
Step 2: A B, and A C are the two functional dependencies that have common left hand side attribute A.
Step 3: The union of right hand sides ends with A BC. Hence, A is the one of the keys for the relation R.


  • Simple.


  • The number of functional dependencies in F+ can be as many as 2n+1 where n is the number of attributes. It costs too much of time.

Featured Content

Multiple choice questions in Natural Language Processing Home

MCQ in Natural Language Processing, Quiz questions with answers in NLP, Top interview questions in NLP with answers Multiple Choice Que...

All time most popular contents

data recovery