The SELECT DISTINCT Statement
The SELECT DISTINCT statement is used to return only distinct (different) values.
Inside a table, a column often contains many duplicate values and sometimes you only want to list the different (distinct) values.
Example
SELECT DISTINCT country FROM customers;
Even though the customers table has 91 records, it only has 21 different countries, and that is what you get as a result when executing the SELECT DISTINCT statement above
SELECT COUNT(DISTINCT)
We can also use the DISTINCT keyword in combination with the COUNT statement, which in the example below will return the number of different countries there are in the customers table.
Example
SELECT COUNT(DISTINCT country) FROM customers;