Question

I have a schema like this

CREATE TABLE TrainManager(
    train_name    VARCHAR(5) REFERENCES Train(name),
    station_id      INT REFERENCES Station(station_id)
);

The two reference tables have an indirect relations.

control (ctrl_id, train_name);

controlremote (ctrl_id, station_id);

As you can, in order to get train name and station id, we need to get into the other two tables to compare ctrl_id besides comparing the train_name and station_id.

$query = "INSERT INTO `train` (train_name, station_id)
    SELECT t.train_name, st.station_id
    FROM train, station
    WHERE t.train_name = ( SELECT c.train_name FROM control c
                           WHERE c.train_name = t.train_name)
            AND
          st.station_id = ( SELECT cr.station_id FROM controlremote cr
                            WHERE cr.station_id = st.station_id)
            AND

But I can’t think of a proper SQL synatx to compare the ctrl_id …

Answer 1

INSERT INTO `train` (train_name, station_id)
SELECT control.train_name, controlremote.station_id
FROM control
LEFT JOIN controlremote ON control.ctrl_id = controlremote.ctrl_id

Something along this way. Just join both tables on ctrl_id. I hope I understood correctly.