Thumbs up / Thumbs down Voting System - php

Question

I'm interesting in creating from scratch such feature to learn, I'm having a hard time understanding and being able to plan such structure. Most programmers from what I've seen
check if someone has already rated the comment or post by their IP address, I dislike that specially due to the fact that each user must be logged in to vote, so why not check the
user iD instead of iP. I also tend to choose performance instead of looks so could someone help me understand how to begin to accomplish this. I've read and searched many posts
on SO(StackOverflow) yet they're not too comprehensive for me. I also read somewhere to have a UNIQUE KEY constraint to only allow one vote, is that correct?
Some suggestions on a table structure and maybe how to design would be greatly appreciated.
I'm confused with such structure, the following tables is what I've got so far.
Posts:
post_iD | message | uid_fk(user id) | voteUp | voteDown
PostsRateSystem:
rate_iD | rate_user_iD(uid_fk from posts table) |
Do I even need a second table to keep track of who has rated.
if(isset($_POST['plus']) && isset($_POST['minus'])){
$plus = $_POST['plus'];
$minus = $_POST['minus'];
$row = insertRate($post_iD, $plus, $minus);
}
<form method="post" action="">
Thumbs Up
Thumbs Down
function insertRate($post_iD, $plus, $minus)
{
if($plus)
{
$sth = $this->db->prepare("UPDATE posts SET voteUp = 1 WHERE post_iD = :postiD");
$sth->execute(array('postiD' => $post_iD));
}
if($minus)
{
$sth = $this->db->prepare("UPDATE posts SET voteDown = 1 WHERE post_iD = :postiD");
$sth->execute(array('postiD' => $post_iD));
}
}

Answer 1

If all users can rate each post once, you'll have to have a third table (votes) that has a userid, a postid and a rating value (either 1 or -1). In that table, you should make the combination of userid and postid unique, because a user is only allowed one vote per post.
To get the total number of votes for a post, you can just sum those values:
SELECT SUM(votes.value) FROM votes WHERE postid = 10
Now, each upvote is counted as one, and each downvote as minus one, because those are the actual values stored.
To save a vote, you'll have to try to update the table first, because the logged in user can already have a vote. If updating fails, probably due to no vote already exists, you can insert a vote for the user for that post.
To speed up things, you can also increment or decrement a vote counter in the post table itself. That one doesn't keep track of all users, but it does keep track of the total score. That way, you don't have to query the sum for each page view of a vote. Keep in mind though, that if a use changes their upvote to a downvote, you'll have to subtract 2 from that score and vice versa. Alternatively, you can use the SELECT SUM query to calculate the votes and store it in the post, after each change to the votes of a post. So after voting for post 10, do this:
UPDATE posts
SET score = (SELECT SUM(value) FROM votes WHERE postid = 10)
WHERE postid = 10