Why Candidate Key is Called a Minimal Super Key?

A key can be referred to as an attribute/a set of attributes that help us identify a row (or tuple) uniquely in a table (or relation). A key is also used when we want to establish relationships between the different columns and tables of a relational database.

Candidate key

• A candidate key is an attribute or a set of attributes that uniquely identify a tuple.
• Candidate keys are defined as a distinct set of attributes from which the primary key can be selected.
• Candidate keys are allowed to have NULL values.
• A candidate key is a minimal super key with no redundant attributes.

Why Candidate Key is Called a Minimal Super Key?

A candidate key is called a minimal super key because we select a candidate key from a set of super keys such that the selected candidate key is the minimum attribute required to uniquely identify the table.

Pre-requisites: Types of Keys

Example-
Let’s take an employee table with attributes Dept_Id, Emp_Id, Name, Aadhar_No, and Email_Id.

Dept_id	Emp_id	Name	Aadhar_No	Email_id
1	1	Satyanshu	9999	satya@gmail.com
2	2	Aditya	8888	aditya@gmail.com
2	3	Avneesh	7777	avneesh@gmail.com
4	4	Aryan	6666	aryan@gmail.com

A super key is a collection of all possible attributes that can be used to uniquely identify the rows in a given relation. 
A super key is a superset of a candidate key.

Super Keys for the above table

{Emp_Id}                                    
{Emp_Id, Aadhar_No}
{Aadhar_No}                            
{Aadhar_No, Email_Id}
{Email_Id}                                 
{Emp_Id, Email_Id}

Candidate keys are as follows

{Emp_Id}                                    
{Aadhar_No}                            
{Email_Id}