How to check if a user has data in certain tables - postgresql

Question

So the approach I'm taking is to create new boolean columns on the user table that if is set to true, then the table has data, if false the table is empty. Now I'm stuck because I don't know how to create the triggers, or more like the procedure that follows the trigger.
So my logic is...for each table have a trigger:
CREATE TRIGGER check_sales_trigger
AFTER INSERT OR DELETE
ON sales
FOR EACH ROW
EXECUTE PROCEDURE check_sales_table();
Then, create a procedure that updates the boolean column on the user table for each table. So basically I need help creating the procedure.
FYI, each client has his own db.

Answer 1

The functions (that's a plural) need to deal with a) new order for user, b) user id change for existing order and c) deleted order. Writing triggers isn't hard, it just needs some reading the manual. No ifs, no buts, no exceptions.
Since this is your first time, here's an example for the more complicated one (because it can lead to deadlocks if poorly written), to get you started:
create function check_sales_table__update() returns trigger as $$
begin
if new.user_id < old.user_id then
update users
set has_sales = true
where id = new.user_id;
update users
set has_sales = exists (select 1 from sales where user_id = old.user_id)
where id = old.user_id;
elsif old.user_id < new.user_id then
update users
set has_sales = exists (select 1 from sales where user_id = old.user_id)
where id = old.user_id;
update users
set has_sales = true
where id = new.user_id;
end if;
return null;
end;
$$ language plpgsql;
(The above assumes a not null field, of course.)