In the context of relational databases, super keys and candidate keys are related concepts, but they have distinct definitions and purposes.

 

Here are the key differences between super keys and candidate keys:

 

Candidate Key:

1. Definition:

   • Unique Identifier: A candidate key is a set of one or more attributes (columns) that uniquely identifies a tuple (row) in a relation (table). Each candidate key must have the property that no two distinct tuples have the same combination of values for the key attributes.

2. Uniqueness:

   • Uniqueness Constraint: Candidate keys ensure that the values in the specified attributes are unique for each tuple in the table.

3. Minimality:

   • Minimal Superkey: A candidate key is a minimal superkey, meaning that if any attribute is removed from the key, it will no longer be unique.

4. Example:

   • Employee ID: In an employee table, the combination of attributes like "Employee ID" and "Email" might be a candidate key if each employee has a unique ID, and no two employees share the same email.

 

Super Key:

1. Definition:

   • Set of Attributes: A super key is a set of one or more attributes that, taken collectively, can uniquely identify a tuple in a relation. It may contain more attributes than necessary to uniquely identify a tuple.

2. Uniqueness:

   • Not Necessarily Unique: Unlike candidate keys, a super key may not be minimal, and it may include additional attributes that do not contribute to uniqueness.

3. Minimality:

   • May or May Not Be Minimal: A super key may or may not be a minimal superkey. It becomes a candidate key if it is both a super key and minimal.

4. Example:

   • Employee Super Key: In the same employee table, a super key could be the combination of "Employee ID," "Name," and "Phone Number." While this uniquely identifies each employee, it is not minimal as removing an attribute would still provide uniqueness.

 

Relationship:

1. Candidate Key vs. Super Key:

   • Subset Relationship: Every candidate key is a super key, but not every super key is a candidate key. Candidate keys are a specific type of super key that is minimal and satisfies the uniqueness constraint.

2. Use in Database Design:

   • Primary Key: Candidate keys are often considered for the role of the primary key, which is the chosen candidate key that will be used to uniquely identify tuples in the table.

3. Uniqueness Constraint:

   • Enforcing Uniqueness: The purpose of both candidate keys and super keys is to enforce the uniqueness constraint in a relational database, ensuring that each tuple is uniquely identified.

 

In summary, a candidate key is a minimal super key with the property of uniqueness, while a super key is any set of attributes that can uniquely identify a tuple, whether minimal or not. Candidate keys are a subset of super keys that meet additional criteria, making them suitable for use as primary keys in a relational database.

 

Thank you.