Exercise

# 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.

Instructions

**100 XP**

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