SELECT DISTINCT

The DISTINCT clause filters out FULL DUPLICATE ROWS. It goes right after the SELECT keyword, since it applies to the entire row, not single columns.

sql

-- select rows if the first name and country of a customer is unique

SELECT DISTINCT country, first_name
FROM Customers;

-- count the unique countries where customers are from 
SELECT COUNT(DISTINCT country)
FROM Customers;

SELECT DISTINCT ​

SELECT DISTINCT