Unique values

Let's assume we have the following table
sales
 
country
city
amount
1
Poland
Warsaw
13
2
Germany
Berlin
24
3
Poland
Katowice
56
And we would like to know all of the countries that the product was sold.
If we use the normal query we know: 
SELECT country from sales it will return 
Poland Germany Poland. This is not what we are looking for because Poland is repeated twice.
To solve it we can use the 
DISTINCT keyword:
SELECT DISTINCT country FROM sales