Few Basic Key of DBMS

a. Primary key – It is a set of one or more fields/columns of a table that uniquely identify a record in database table. It cannot accept null, duplicate values. Only one Candidate Key can be Primary Key.

b. Candidate key – It is a set of one or more fields/columns that can identify a record uniquely in a table. There can be multiple Candidate Keys in one table. Each Candidate Key can work as Primary Key.

Eg: In below diagram ID, Roll No. and Enroll No. are Candidate Keys since all these three fields can be work as Primary Key.

c. Alternate key – It is a key that can be work as a primary key. Basically, it is a candidate key that currently is not primary key.

Eg: In below diagram Roll No. and Enroll No. becomes Alternate Keys when we define ID as Primary Key.

 

ID	Roll No.	Name	Enroll No.	Address	Dept ID
1	6068741	Harsh	ADM1	Delhi	1
2	6068742	Neha	ADM2	Noida	1
3	6068743	Aditi	ADM3	Gurgaon	2

Primary Key

                             Alternate Keys

Possible Candidate Keys