How to set a trigger in PostgreSQL? - postgresql

Question

I have the tables show here -> How can I display if an airline departs from all the airport in my DB in Postgresql?
I've update the airlines table like below
ALTER TABLE airlines
ADD COLUMN count_flight INTEGER;
Now I need to update the column count_flight with the number of flight for each airline that are already stored in volo table.
How can I set a trigger that do what I need + update the value of count_flight every time I insert a new row in volo table?
EDIT
I've set a trigger like below and it works
CREATE FUNCTION update_flight() RETURNS TRIGGER AS $updateN_voli$
DECLARE
airline_name varchar;
BEGIN
IF (TG_OP = 'INSERT') THEN
airline_name = NEW.airline;
UPDATE airlines
SET count_flight = count_flight + 1
WHERE airline_name = airlines.airline_name;
END IF;
RETURN NULL;
END;
$updateN_voli$ LANGUAGE plpgsql;
/* Creo il trigger */
CREATE TRIGGER update_flight
AFTER INSERT ON volo
FOR EACH ROW
EXECUTE PROCEDURE update_flight();
Now I can't figure out how can I update the value with the existing flight. As I said in the comment below I have found a possible solution but is a bit complex if I have a big amount of row in table volo.

Answer 1

I've found the answer:
This is for update the column with existing data in volo table:
DO
$$
DECLARE
a varchar;
BEGIN
FOR a IN select airline from airlines LOOP
UPDATE airlines
SET count_flight = (SELECT COUNT (airline) FROM volo WHERE airline = a)
WHERE airline = a;
END LOOP;
END;
$$;
This is for update te airlines table every time a new flight is add to volo table:
CREATE FUNCTION update_newVoli() RETURNS TRIGGER AS $updateN_voli$
DECLARE
name_airline varchar;
BEGIN
IF (TG_OP = 'INSERT') THEN
name_airline = NEW.airline;
UPDATE compagnie
SET count_flight = count_flight + 1
WHERE name_airline = airlines.airline;
END IF;
RETURN NULL;
END;
$updateN_voli$ LANGUAGE plpgsql;
CREATE TRIGGER update_newVoli
AFTER INSERT ON volo
FOR EACH ROW
EXECUTE PROCEDURE update_newVoli();