Postgres trigger that affects rows with certain values - sql

Question

I'm postgres newbie and i have a question regarding triggers. Lets say we have 2 tables. One with cities and one with people.
CITIES:
PEOPLE:
I need a trigger that updates 'processed' value in table cities from 0 to 1 when 'processed' value in table people updates from 0 to 1 for all people from that city.
For example: John, Ian and Claire are from Berlin and when 'processed' value is updated to 1 for each of them, then the trigger updates processed value for Berlin in table cities to 1.
What is the best way to do this?

Answer 1

This will work:
CREATE OR REPLACE FUNCTION mytrigger() RETURNS TRIGGER AS $mytrigger$
BEGIN
IF (SELECT EXISTS (SELECT 1 FROM PEOPLE WHERE city_id = NEW.city_id AND processed = '0')) THEN
UPDATE CITIES SET processed = '0' WHERE id = NEW.city_id;
RETURN NEW;
ELSE
UPDATE CITIES SET processed = '1' WHERE id = NEW.city_id;
RETURN NEW;
END IF;
END;
$mytrigger$ LANGUAGE plpgsql;
CREATE TRIGGER mytrigger
AFTER UPDATE ON PEOPLE
FOR EACH ROW EXECUTE PROCEDURE mytrigger();

Answer 2

try this query:
with c as (
select city_id
from people
where name = NEW.name
)
, a as (
select avg(processed)
from people
join c on c.city_id = people.city_id
)
update cities u
set processed = 1
from a
where u.id = a.city_id
and avg = 1
alias c gets city id, then a aggregates processed for that sict id in people and lastly update updates only when all names are processed.