How do I undo a submition after the value has been sent to database with ajax?

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PHP

Ajax

jQuery

database

I am implementing a like and unlike system to my project. I followed this tutorial to make things works. I trying to make an undo like/dislike function if I click again on the same button, but I can't figure out, what should I modify or add to codes.

What I have tried:

I have tried to check if the record is exist in the database then if yes, delete it, but nothing happens:

$query = "SELECT * FROM like_unlike WHERE type='1' and userid = '$userid' and postid='$postid'";
$result = mysqli_query($conn,$query);
$fetchliked = mysqli_fetch_array($result);
$liked = $fetchliked['type'];
if ($liked == '1') {
    $deletequery = "DELETE FROM like_unlike WHERE type='1' userid='$userid' and postid='$postid'";
    mysqli_query($conn,$deletequery);
}