Up/Down voting system for WordPress

Looking in all posts with keywords “voting system, upvote, downvote”, I can’t find a satisfying solution for this issue, except accepting one of the available plugins, spend a lot of time undersanding it and try to modify its functionnalies so that they fit with the requirements. So, I am convinced in writing my own code to create a stack-exchange-like voting system for WordPress. Without your suggestions I cannot find the starting point for the main part of the skeleton of my code.

My actual approach :

• Create two custom fields: the first stores the number of post
  upvotes, and the second, the number of downvotes. It is possible to display votes state using
  get_post_meta function, and change vote state from front end using update_post_meta.
• Find a way to store in the database the voted posts by a specific user and the state
  of vote(up, down or nothing). So that logged in users see if they already voted.

If (user is logged in )
{
Show the upvote or downvote state already done by this user;
Allow voting ;
}
else 
{
don't allow voting;
}

My question is:
How to resolve the second issue. Is there a way to achieve it using the customizing features offered by WordPress PHP functions (without using SQL queries, since I don’t know how to use SQL). I observe that all the Up/Down voting plugins use SQL queries in their code which make me really scared. Should I learn SQL to achieve my goal?

A simple user meta row can handle that for you (the second issue), you can store the post id and the vote (up/down) in an array and that is just the same as post meta ex

/**
 * update user vote per post
 * @param  int $user_id 
 * @param  int $post_id 
 * @param  mixed $vote can be an integer 1 / -1 and can also be a string "up"https://wordpress.stackexchange.com/"down"
 * @return void
 */
function update_user_vote($user_id,$post_id,$vote){
    //get all saved user data    
    $user_votes = get_user_meta($user_id,'updown_votes',true);
    $user_votes[$post_id] = $vote;
    //update with the new vote
    update_user_meta($user_id,'updown_votes',$user_votes);
}

/**
 * get user vote of post
 * @param  int $user_id 
 * @param  int $post_id 
 * @return if user has voted the his vote will be returned , zero if no vote found for post by this user
 */
function get_user_vote($user_id,$post_id){
    //get all saved user data    
    $user_votes = get_user_meta($user_id,'updown_votes',true);
    return (isset($user_votes[$post_id])) ?  $user_votes[$post_id] : 0;
}

and all done with no SQL 🙂