Show me the official names of the cities that have hosted only one competition?
SELECT T1.Official_Name FROM city AS T1 JOIN farm_competition AS T2 ON T1.City_ID   =   T2.Host_city_ID GROUP BY T2.Host_city_ID HAVING COUNT ( * )    =   1
official names of the cities = T1.Official_Name; hosted only one competition = COUNT ( * ) = 1;