Past Year - 2 Mark Questions

Computer Science - Class 12
Chapter 8 - Database Concepts

## (ii) primary key

• Candidate key refers to the set of all attributes which can uniquely identify tuples in a relation.
• Primary key refers to a set of one or more attributes which can uniquely identify tuples in a relation.
• A table can have only one primary key but a table can have multiple candidate keys.

Example:

 Employee_id Employee_name Department Emp_Email 101 Ravi Kumar Sales ravi1@gmail.com 102 Tarun A IT ta121@gmail.com 103 Jennifer Davis HR jenny@gmail.com 104 Martin Woods HR martin@gmail.com 105 Ron Romy IT roro@gmail.com

In the EMPLOYEE table given above,

• Employee_id and Emp_Email are the set of attributes which can uniquely identify the tuples in the table. So, the candidate keys of table EMPLOYEE are Employee_id and Emp_Email.
• Since a table can have only one primary key, we can select either Employee_id or Emp_Email as the primary key .

In the table given in the question, Id and Product are 2 attributes which can uniquely identify the tuples.

So,

Candidate keys: Id, Product

Primary key: Id

### Transcript

Observe the following table carefully and write the names of the most appropriate columns, which can be considered as (i) candidate keys and (ii) primary key Answer: Candidate key refers to the set of all attributes which can uniquely identify tuples in a relation. Primary key refers to a set of one or more attributes which can uniquely identify tuples in a relation. A table can have only one primary key but a table can have multiple candidate keys. Example: Employee_id Employee_name Department Emp_Email 101 Ravi Kumar Sales ravi1@gmail.com 102 Tarun A IT ta121@gmail.com 103 Jennifer Davis HR jenny@gmail.com 104 Martin Woods HR martin@gmail.com 105 Ron Romy IT roro@gmail.com In the EMPLOYEE table given above, Employee_id and Emp_Email are the set of attributes which can uniquely identify the tuples in the table. So, the candidate keys of table EMPLOYEE are Employee_id and Emp_Email. Since a table can have only one primary key, we can select either Employee_id or Emp_Email as the primary key. In the table given in the question, Id and Product are 2 attributes which can uniquely identify the tuples. So, Candidate keys: Id, Product Primary key: Id