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.

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

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Exercise

Identify keys with SELECT COUNT DISTINCT

Instructions