wp list all users who haven't commented on a post

Question

I'm trying find a way in my single.php to list all users (display name) that haven't commented on the current post.

I have this snippet that lists all the users who have commented...

/**
 * Retrieve a list of comment authors' names.
 *
 * @param  int    $id  Optional. Post ID.
 * @param  string $sep Optional. List separator.
 * @return string|null Names list.
 */
function get_comment_authors_list( $id = 0, $sep = ', ' ) {
    $post_id = $id ? $id : get_the_ID();
    if ( $post_id ) {
        $comments = get_comments( array(
            'post_id' => $post_id,
            'status'  => 'approve',
            'type'    => 'comment',
        ) );

        $names = array();
        foreach ( $comments as $comment ) {
            $name = $comment->comment_author;
            if ( $comment->user_id ) {
                $user = get_userdata( $comment->user_id );
                $name = $user ? $user->first_name : $name;
            }

            $arr = explode( ' ', trim( $name ) );
            if ( ! empty( $arr[0] ) && ! in_array( $arr[0], $names ) ) {
                $names[] = $arr[0];
            }
        }
        unset( $comments );

        $sep = $sep ? $sep : ', ';
        return implode( $sep, $names );
    }
}

/**
 * Display a list of comment authors' names.
 *
 * @param  int    $id  Optional. Post ID.
 * @param  string $sep Optional. List separator.
 * @return null
 */
function the_comment_authors_list( $id = 0, $sep = ', ' ) {
    echo get_comment_authors_list( $id, $sep );
}

/*
 * Available parameters for the Shortcode:
 *   int    post_id  Optional. Post ID.
 *   string list_sep Optional. List separator.
 *
 * Usage examples:
 *   [comment_authors_list /]
 *   [comment_authors_list post_id="1" list_sep=" · " /]
 */
add_shortcode( 'comment_authors_list', 'comment_authors_list_shortcode' );

/**
 * Shortcode callback function.
 *
 * @see get_comment_authors_list()
 * @return string|null Names list.
 */
function comment_authors_list_shortcode( $atts = array() ) {
    $atts = shortcode_atts( array(
        'post_id'  => 0,
        'list_sep' => '',
    ), $atts );

    return get_comment_authors_list( $atts['post_id'], $atts['list_sep'] );
}

Answer 1

OK, so you want to get set complements from MySQL.

One way to do this will be with query like that:

SELECT u.ID
FROM
    wp_users  u
    LEFT OUTER JOIN wp_comments c ON
        (u.ID = c.user_id AND comment_post_ID = <YOUR_POST_ID>)
WHERE c.user_id is null

But you should remember, that such query won't be very efficient. So it will be OK, if your DB isn't very big. You should think of some other solution otherwise.