Show station ids for stations located in London.
select t4.station_id from train_station as t3 join station as t4 on t3.station_id = t4.station_id where t4.location = "London"
Show the names for all trains.
select t2.name from train_station as t1 join train as t2 on t1.train_id = t2.train_id
Only show the names for the trains that do not pass any station located in London.
select t2.name from train_station as t1 join train as t2 on t1.train_id = t2.train_id where t1.station_id not in (select t4.station_id from train_station as t3 join station as t4 on t3.station_id = t4.station_id where t4.location = "London")