WP Query Orderby Taxonomy Term Name

functions.php

add_filter('posts_clauses', 'posts_clauses_with_tax', 10, 2);
function posts_clauses_with_tax( $clauses, $wp_query ) {
	global $wpdb;
	//array of sortable taxonomies
	$taxonomies = array('example-taxonomy', 'other-taxonomy');
	if (isset($wp_query->query['orderby']) && in_array($wp_query->query['orderby'], $taxonomies)) {
		$clauses['join'] .= "
			LEFT OUTER JOIN {$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)
		";
		$clauses['where'] .= " AND (taxonomy = '{$wp_query->query['orderby']}' OR taxonomy IS NULL)";
		$clauses['groupby'] = "rel2.object_id";
		$clauses['orderby']  = "GROUP_CONCAT({$wpdb->terms}.name ORDER BY name ASC) ";
		$clauses['orderby'] .= ( 'ASC' == strtoupper( $wp_query->get('order') ) ) ? 'ASC' : 'DESC';
	}
	return $clauses;
}

Kudos. Needed this badly and it works likes a charm.

Thank you so much ! I spent hours searching for a solution, some of what I found on the net was really disgusting : aggregation of multiple query results (one by taxonomy term), retrieving all posts to sort them and then apply a pagination in PHP, etc...
This is by far the cleanest solution ! I had to twist it a bit of course, but you saved me a lot of time. Thanks again !

Great, thanks a lot.

In case anyone is trying to order by integer and you want 1,2,3,4 instead of 1,10,11,12 etc replace:

$clauses['orderby'] = "GROUP_CONCAT({$wpdb->;terms}.name ORDER BY name ASC) ";

with

$clauses['orderby'] = "CAST(GROUP_CONCAT({$wpdb->terms}.name ORDER BY name ASC) as DECIMAL) ";

+1 works great.