Postgresql trigger to calculate total score - sql

Question

I am trying to learn how to create a trigger in postgresql. I have a table
Thread_user - table name
thread_id
user_id
points
Thread - table name
thread_id
total_points
I want to on update of any thread_user row to update the total points in the thread table. I need to basically select * from thread_user where thread_id = thread_id of inserted item and then add the points then update thread_points in the threads table. I believe this is done in triggers but maybe a stored procedure would be better.

Answer 1

First step is to make a function which calculates the sum of points and updates a row in the calculated_points table.
Thereafter you'll ahve to create a trigger which is called upon inserting a row in the user_points table.
DROP TABLE IF EXISTS user_points CASCADE;
CREATE TABLE user_points (
id SERIAL PRIMARY KEY,
user_id INT NOT NULL,
points INT NOT NULL
);
DROP TABLE IF EXISTS calculated_points CASCADE;
CREATE TABLE calculated_points (
id SERIAL PRIMARY KEY,
user_id INT NOT NULL UNIQUE,
points INT NOT NULL
);
INSERT INTO calculated_points (user_id, points)
VALUES
(1, 0),
(2, 0);
CREATE OR REPLACE FUNCTION calculate_total_points()
RETURNS trigger AS $calculate_total_points$
BEGIN
UPDATE calculated_points
SET points = (SELECT SUM(points)
FROM user_points
WHERE user_id = NEW.user_id)
WHERE user_id = NEW.user_id;
RETURN NEW;
END;
$calculate_total_points$ LANGUAGE plpgsql;
CREATE TRIGGER points_added
AFTER INSERT
ON user_points
FOR EACH ROW
EXECUTE PROCEDURE calculate_total_points();