wordpress-wp-query-order-by-taxonomy-terms-names.php

<?php

add_filter('posts_clauses', 'hwk_wp_query_order_by_taxonomy_terms', 10, 2);
function hwk_wp_query_order_by_taxonomy_terms($clauses, $wp_query){
    global $wpdb;
    
    if(!isset($wp_query->query['orderby']) || strpos($wp_query->query['orderby'], 'tax_') !== 0)
        return $clauses;

    $taxonomy = substr_replace($wp_query->query['orderby'], '', 0, strlen('tax_'));
    
    $clauses['join'] .= "
        LEFT OUTER JOIN {$wpdb->term_relationships} AS rel2 ON {$wpdb->posts}.ID = rel2.object_id
        LEFT OUTER JOIN {$wpdb->term_taxonomy} AS tax2 ON rel2.term_taxonomy_id = tax2.term_taxonomy_id
        LEFT OUTER JOIN {$wpdb->terms} USING (term_id)
    ";
    
    $clauses['where'] .= " AND (taxonomy = '{$taxonomy}' OR taxonomy IS NULL)";
    $clauses['groupby'] = "rel2.object_id";
    $clauses['orderby'] = "GROUP_CONCAT({$wpdb->terms}.name ORDER BY name ASC) " . (strtoupper($wp_query->get('order')) == 'ASC') ? "ASC" : "DESC";
    
    return $clauses;
}

// Usage:
// 
// $query = new WP_Query(array(
//     'tax_query' => array(
//         array(
//             'taxonomy'  => 'category',
//             'field'     => 'slug',
//             'terms'     => 'my-category'
//         )
//     ),
//     'orderby'   => 'tax_category' // Taxonomy: category
//     'order'     => 'DESC'
// ));