Best way to register votes?

So in the website there are posts, and users can vote them. So there is an Id from the post and an Id from the user. Database is MySQL.

I'm pretty shure what I'm doing is worng.

I only have a varchar(22), and in it I put for exemple 233.788, that is: post id "." user id

Otherways if I have two colums one for each id, it will have to search all the id from a post and then see if there is the other id of the user. I suppose this is the correct way perhaps faster seraching INT's, but using the other I just make and insert and if it fails that probably means the vote alredy exist because is unique key, and with only one simple query I do the job. With the correct methot I need a more complex query.

I'm afraid I not explained it well so some exemples:

What I'm donig:

$vot_id = $post_id.".".$user_id;
$sql = "INSERT INTO vot_post (vot_pos) VALUE ('$vot_id')";

In the database: "varchar(22)" Unique Key. Exemple: 233.788


What I think I should do but I really don't know If will be better or faster:

$post_id = 233;
$user_id = 788;
$sql = "INSERT INTO vot_post(id_post,id_user) SELECT '$post_id', '$user_id' FROM DUAL WHERE NOT EXISTS (SELECT id_post FROM vot_post WHERE id_post='$post_id' AND id_user='$user_id' LIMIT 1)";
In the database: "INT(10)" and "INT(12)"



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SELECT id_post FROM vot_post WHERE id_post = 233 AND id_user = 788;

Then test for a row being returned. If one was, that user voted on that post already. (I'm assuming that the table is named vot_post.)

The database will run just about as fast searching one field or two fields, it doesn't search first one, then the other.