Skip to content
Related Articles
Open in App
Not now

Related Articles

Difference between Super Key and Candidate Key

Improve Article
Save Article
  • Difficulty Level : Medium
  • Last Updated : 12 Jul, 2022
Improve Article
Save Article

Super Key: Super Key is an attribute (or set of attributes) that is used to uniquely identifies all attributes in a relation. All super keys can’t be candidate keys but the reverse is true. In relation, a number of super keys is more than a number of candidate keys. 

Prerequisite – Keys in Relational Model 

Example: We have a given relation R(A, B, C, D, E, F) and we shall check for super keys by following given dependencies: 
 

Functional dependencies         Super key
AB->CDEF                         YES
CD->ABEF                         YES
CB->DF                           NO
D->BC                            NO 

By Using key AB we can identify the rest of the attributes (CDEF) of the table. Similarly, Key CD. But, by using key CB we can only identify D and F, not A and E. Similarly key D

Candidate Key: A candidate key is a set of attributes (or attributes) that uniquely identify the tuples in relation to or table. As we know the Primary key is a minimal super key, so there is one and only one primary key in any relationship but there is more than one candidate key that can take place. The candidate key’s attributes can contain a NULL value which opposes to the primary key. 

Example:  

Student{ID, First_name, Last_name, Age, Sex, Phone_no} 

Here we can see the two candidate keys ID and {First_name, Last_name, DOB, Phone_no}. So here, there are present more than one candidate keys, which can uniquely identify a tuple in a relation. 

Difference between Super Key and Candidate Key:  

Super KeyCandidate Key
Super Key is an attribute (or set of attributes) that is used to uniquely identifies all attributes in a relation.Candidate Key is a subset of a super key.
All super keys can’t be candidate keys.But all candidate keys are super keys.
Various super keys together makes the criteria to select the candidate keys.Various candidate keys together makes the criteria to select the primary keys.
In a relation, number of super keys is more than number of candidate keys.While in a relation, number of candidate keys are less than number of super keys.
Super key attributes can contain NULL values.Candidate key attributes can also contain NULL values.
My Personal Notes arrow_drop_up
Related Articles

Start Your Coding Journey Now!