Candidate Key in DBMS

In Database Management Systems (DBMS), a candidate key is a set of one or more attributes (columns) that can uniquely identify a record (row) in a table. A table can have multiple candidate keys, but only one of them is chosen as the primary key.

Key Points about Candidate Key:

  1. Uniqueness: Every candidate key must be able to uniquely identify each row in the table.

  2. No Redundancy: A candidate key is a minimal superkey with only essential attributes for uniqueness.

  3. Multiple Candidate Keys: A table can have more than one candidate key. The user chooses one of them to be the primary key. The others are called alternate keys.

How to Find the Candidate Key in DBMS?

Answer: We can find all possible candidate keys by using the closure method, which is applied on the given Relational table and functional dependencies.

Examples of Candidate Keys in DBMS

Consider a Student table with the following columns:

Student_ID Email Phone Name
1 john@email.com 1234567890 John
2 jane@email.com 9876543210 Jane

  • Student_ID: Can uniquely identify each student.

  • Email: Can also uniquely identify each student.

  • Phone: Similarly, it can uniquely identify each student.

In this case, Student_ID, Email, and Phone are all candidate keys because each of them can uniquely identify a student. However, one of them would be chosen as the primary key.

Difference Between Candidate Key and Primary Key

  • Candidate Key: A set of attributes that can uniquely identify a record in a table. There can be multiple candidate keys.

  • Primary Key: A candidate key chosen to uniquely identify records in a table. It cannot contain null values.

The primary key is selected from one of the candidate keys, and it’s used to maintain uniqueness within a table. While a table can have multiple candidate keys, only one is designated as the primary key.