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
