CREATE TABLE train (train_id NUMBER, name CLOB, time CLOB, service CLOB)
CREATE TABLE train_station (train_id NUMBER, station_id NUMBER)
CREATE TABLE station (station_id NUMBER, name CLOB, annual_entry_exit NUMBER, annual_interchanges NUMBER, total_passengers NUMBER, location CLOB, main_services CLOB, number_of_platforms NUMBER)

Show all locations with only 1 station.
SELECT location FROM station GROUP BY location HAVING COUNT(*) = 1
SELECT "location" FROM "station" GROUP BY "location" HAVING COUNT(*) = 1