Order posts by taxonomy terms name

filter_posts_by_taxonomy_terms_name.php

<?php

add_filter('posts_clauses', function (array $clauses, WP_Query $wp_query): array {
    $wpdb = $GLOBALS['wpdb'];
    
    $taxonomy = 'course_level';

    if (! isset($wp_query->query['orderby']) || $taxonomy !== $wp_query->query['orderby']) {
        return $clauses;
    }

    $clauses['join'] .= <<<SQL
LEFT OUTER JOIN {$wpdb->term_relationships} ON {$wpdb->posts}.ID={$wpdb->term_relationships}.object_id
LEFT OUTER JOIN {$wpdb->term_taxonomy} USING (term_taxonomy_id)
LEFT OUTER JOIN {$wpdb->terms} USING (term_id)
SQL;

    $clauses['where'] .= " AND (taxonomy = '{$taxonomy}' OR taxonomy IS NULL)";
    $clauses['groupby'] = 'object_id';
    $clauses['orderby'] = "GROUP_CONCAT({$wpdb->terms}.name ORDER BY name ASC) ";
    $clauses['orderby'] .= 'ASC';

    return $clauses;
}, 10, 2);