Example WordPress plugin to hook into the 'wpbcr_shortcode_best_comments_sql' filter of the WP-Comment Rating Plugin by WP-Buddy

my-shortcode-ranking-filter.php

<?php
/*
Plugin Name: My Shortcode Ranking Filter for WP-Comment Rating Plugin
Description: Hooks into the sql filter of the WP-Comment-Rating and allows the usage of the shortcode like this: [wpbcr_comment_ranking do="filter_by_xy_meta"]
*/

add_filter( 'wpbcr_shortcode_best_comments_sql', 'my_wpbcr_shortcode_best_comments_sql', 10, 6 );


/**
 * The function called by the 'wpbcr_shortcode_best_comments_sql' filter and
 * outputs a new SQL query depending on a comment meta value and meta key.
 *
 * Allows the usage of the shortcode like this: [wpbcr_comment_ranking do="filter_by_xy_meta"].
 * Where the meta key is 'some_keyval' and the meta value is 'test' in this script.
 *
 * ATTENTION: This only works if the WP-Comment-Plugin version >= 1.2 is used.
 *
 * @param string $sql
 * @param string $where
 * @param string $sort_by
 * @param int    $limit
 * @param int    $post_id
 * @param string $do
 *
 * @return string
 */
function my_wpbcr_shortcode_best_comments_sql( $sql, $where, $sort_by, $limit, $post_id, $do ) {

	/*
	 * stop here if the 'do' variable is not 'filter_by_xy_meta'
	 *
	 * ATTENTION: replace the name 'filter_by_xy_meta' with your own value. (It is then used in the shortcode).
	 */
	if ( 'filter_by_xy_meta' != $do ) {
		return $sql;
	}

	/**
	 * @var wpdb $wpdb
	 */
	global $wpdb;

	// stop here if - in any case - the wpdb does not exist.
	if ( ! is_a( $wpdb, 'wpdb' ) ) {
		return $sql;
	}

	/*
	 * add a query to only return rows that have the 'meta_key' = 'some_value'
	 * where c is the original wp_comments table.
	 *
	 * ATTENTION: replace 'some_keyval' with your meta_key name.
	 */
	$query_add = ' LEFT JOIN ' . $wpdb->commentmeta . ' as my_q ON ( my_q.comment_ID = cm.comment_id AND my_q.meta_key = "some_keyval" )';

	// hook in just before the WHERE clause.
	$sql = str_replace( 'WHERE', $query_add . ' WHERE ', $sql );

	/*
	 * hook into the WHERE clause.
	 *
	 * ATTENTION: replace 'test' with your own meta value.
	 */
	$sql = str_replace( 'WHERE', ' WHERE my_q.meta_value = "test" AND ', $sql );

	return $sql;
}

Copy this into a file maybe called 'my-shortcode-ranking-filter.php' and put it into the /wp-contents/plugins/ folder of WordPress. Activate it from the administration panel. Use the shortcode like this:

[wpbcr_comment_ranking do="filter_by_xy_meta"]