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