In a GROUP BY statement only distinct values are shown for the column in the GROUP BY.
This example shows the continents hosting the Olympics with the count of the number of games held.

SELECT continent, COUNT(yr) FROM games GROUP BY continent
DROP TABLE games; CREATE TABLE games(yr INT, city VARCHAR(20),continent VARCHAR(20)); INSERT INTO games(city,yr,continent) VALUES ('Sydney',2000,'Australasia'); INSERT INTO games(city,yr,continent) VALUES ('Athens',2004,'Europe'); INSERT INTO games(city,yr,continent) VALUES ('Beijing',2008,'Asia'); INSERT INTO games(city,yr,continent) VALUES ('London',2012,'Europe');

SELECT continent, COUNT(yr) FROM games
 GROUP BY continent