But what if we want to know, for example, unique pairs 'country-city' of our site visitors? Surely, we can extract unique countries (like in the previous chapter) and unique cities. But then we have to connect each city to the country manually.

This task can be solved by the same DISTINCT, but this time with two columns. For example,

The DISTINCT statement will return all the unique combinations of columns you chose. Using parentheses is not mandatory, i.e. you for the example above you can write DISTINCT country, city. But placing multiple columns within DISTINCT parentheses will return only one column of all unique combinations in (country, city) format.