SQL Upvote Downvote system - php

Question

I am currently working on a forum where posts need to have an upvote/downvote system.
My current sql(phpmyadmin) structure is like this:
Table 1 (posts)
| post_id | post_title | post_score |
Table 2 (pvotes)
| pvote_id | fk_post_id | fk_user_id | pvote_score |
I want to somehow make post_score (in table 1), find all pvote_score (table 2) columns and add/subtract them together, where fk_post_id (table 2) is = to post_id (table 1)
This way I hope to make a voting system that only allows every user to vote once, and automatically calculate a posts post_score from the pvote_score values.
EXAMPLE:
user_1 upvotes post_1 inserting following into table 2:
| (pvote_id) 1 | (fk_post_id) 1 | (fk_user_id) 1 | (pvote_score) 1 |
I now want post_score (table 1) to find all entries in table 2 where:
fk_post_id is same as post_id, and thereafter add or subtract the values from pvote_score and make the sum the NEW value of post_score.
I am trying to make it work just like stackoverflows own upvote/downvote system.
EDIT 1:
Question:
I want to know how can I make the post_score column automatically add/subtract the values from pvotes_score and show the sum as its value?

Answer 1

I've created your database structure like this:
CREATE TABLE `posts` (
`post_id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`post_title` varchar(50) DEFAULT NULL,
`post_score` int(11) DEFAULT NULL,
PRIMARY KEY (`post_id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=latin1;
INSERT INTO `posts` VALUES (NULL, 'test', 0), (NULL, 'test2', 0);
CREATE TABLE `pvotes` (
`pvote_id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`fk_post_id` int(11) DEFAULT NULL,
`fk_user_id` int(11) DEFAULT NULL,
`pvote_score` int(11) DEFAULT NULL,
PRIMARY KEY (`pvote_id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=latin1;
INSERT INTO `pvotes` VALUES (NULL, 1, 0, 2), (NULL, 1, 0, 3), (NULL, 1, 0, -1), (NULL, 2, 0, 2);
This is the query that should do the trick:
UPDATE posts SET post_score = (SELECT SUM(pvote_score) FROM pvotes WHERE fk_post_id = post_id);
The result I've got is this:
post_id | post_title | post_score
1 | test | 4
2 | test2 | 2