Archive for September 17, 2015

DB Basics – As Primary Key can identify a row uniquely, what are the scenarios you need a Super Key?

September 17, 2015 Leave a comment

In one of my [previous post] I discussed about various types of Keys in DBMS, like Candidate, Primary, Composite & Super Keys.

I got few questions like:
1. If there is already a Primary Key to identify a row uniquely, then in which scenario a Super Key can use used?
2. How to create a Super Key like a Primary Key in SQL Server or Oracle?

–> First of all what is a Primary Key (PK): It is the Candidate key attribute/column that is most suited to maintain uniqueness in a table at the tuple/row level.

–> And a Super Key is a super-set of Candidate key. If you add any other attribute/column to a Candidate Key then it become a Super Key. Or a minimal Super Key is called a Candidate Key.

–> Now consider the same example that I discussed in my [previous post]:

    DOB, -- Date of birth
    DOJ, -- Date of joining
    SSN, -- Social Security Number
    DeptID, -- Department ID
    MgrID -- Manager ID

Now in this table there are 2 Candidate Keys i.e. EmployeeID & SSN.
– EmployeeID: should be unique with auto-increment column.
– SSN: should also be unique for every employee.

So, here you cannot make SSN as a Primary Key, because its a PII and secure data.
Hence, creating EmployeeID as Primary Key makes sense, this way SSN will be left as an Alternate Key.

–> Now, coming to the Super Key concept: As per the above definition, a minimal Super Key will be called as a Candidate or Primary Key. Thus every Primary Key and Alternate Key is also a Super Key. And if you combine any other column with the EmployeeID (PK) column, the combination will also be called as a Super Key, like:
– EmployeeID
– EmployeeID + DOB
– EmployeeID + DOJ
– EmployeeID + DeptID + MgrID
– EmployeeID + DOB + DOJ + DeptID + MgrID
– EmployeeID + [other column combinations]

For all the above and other possible combinations the row will still be identified uniquely, thus all these combinations will act like a Super Key.

And, if you add all columns with the EmployeeID, it is called a Trivial Super Key, because it uses all columns of Employee table to identify a row.

And in SQL Server or Oracle there is no syntax as such to create a Super Key, this is just a concept in DBMS.

Hope this helps !!!

Categories: DB Concepts Tags: ,