Candidate Key in DBMS

First, identify all the super keys present in the table, then eliminate the super keys that contain a column with duplicate data. Then the remaining superkeys  that are left or nothing but candidate keys

1. {Id}: ID column will contain all unique values hence ID column is a candidate key
2. {phone}: As no two students have the same phone number, it is not a redundant data column and  hence phone column is a candidate key
3. {Id, phone}: As both ID and phone are unique for all students this combination is a valid candidate key
4. {Id, Name}: This combination is not a candidate key because the name column may have duplicate values
5. {Id, phone, Name}: This combination is not a candidate key because the name column may have duplicate values
6. {Name, Phone}: This combination is not a candidate key because the name column may have duplicate values

Candidate keys available in the table  student

• {Id}
• {phone}
• {Id, phone}

Candidate key versus super key

• First of all, you need to understand that all candidate keys are super keys this is because candidate keys are selected from super keys
• Look for those keys from which we can remove any columns that contain duplicate data. In the above example, we have not chosen {Id, name} as candidate key because {Id} alone can identify a unique row in the table, and {Name} column is redundant.