Identify keys with SELECT COUNT DISTINCT

There's a very basic way of finding out what qualifies for a key in an existing, populated table:

Count the distinct records for all possible combinations of columns. If the resulting number x equals the number of all rows in the table for a combination, you have discovered a superkey.
Then remove one column after another until you can no longer remove columns without seeing the number x decrease. If that is the case, you have discovered a (candidate) key.

The table professors has 551 rows. It has only one possible candidate key, which is a combination of two attributes. You might want to try different combinations using the "Run code" button. Once you have found the solution, you can submit your answer.

This exercise is part of the course

Introduction to Relational Databases in SQL

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Exercise instructions

Using the above steps, identify the candidate key by trying out different combination of columns.

Hands-on interactive exercise

Have a go at this exercise by completing this sample code.

-- Try out different combinations
___ COUNT(___(___)) 
FROM professors;

Edit and Run Code