What were the themes of each of those competitions?
SELECT T1.Official_Name, T2.Theme 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
themes of competitions refers to T2.Theme; each of those competitions indicates a JOIN condition between tables; competitions hosted in each city once refers to HAVING COUNT(*) = 1;