Definition: A super key is a set or one of more columns (attributes) to uniquely identify rows in a table. Often people get confused between super key and candidate key, so we will also discuss a little about candidate key here.
How candidate key is different from super key?
Answer is simple – Candidate keys are selected from the set of super keys, the only thing we take care while selecting candidate key is: It should not have any redundant attribute. That’s the reason they are also termed as minimal super key.
Let’s take an example to understand this: Employee table
| Emp_SSN | Emp_Number | Emp_Name |
| 123456789 | 226 | Steve |
| 999999321 | 227 | Ajeet |
| 888997212 | 228 | Chaitanya |
| 777778888 | 229 | Robert |
Super keys:
- {Emp_SSN}
- {Emp_Number}
- {Emp_SSN, Emp_Number}
- {Emp_SSN, Emp_Name}
- {Emp_SSN, Emp_Number, Emp_Name}
- {Emp_Number, Emp_Name}
All of the above sets are able to uniquely identify rows of the employee table.
Candidate Keys:
As I stated above, they are the minimal super keys with no redundant attributes.
- {Emp_SSN}
- {Emp_Number}
Only these two sets are candidate keys as all other sets are having redundant attributes that are not necessary for unique identification.
Primary key:
Primary key is being selected from the sets of candidate keys by database designer. So Either {Emp_SSN} or {Emp_Number} can be the primary key.
Why Emp_Name is not candidate or super key ?
Emp_Name cannot be both because names are not unique e.g. there could be hundreds of Jack inside the database.
sir why don’t Emp_Number is a super key
Emp_Number is a super key, in fact with just Emp_Number we can lead up to Emp_Name or Emp_SSN;
Emp_Name is NOT a super key because we can have 2 “Steve” or 3 or 4 in that table.. Name are not unique, we cannot say the same for SSN codes or Emp_Number. Bye
Gabriele