Get all distinct values of a meta field in Wordpress

get_meta_values.php

function _get_all_meta_values($key) {
    global $wpdb;
	$result = $wpdb->get_col( 
		$wpdb->prepare( "
			SELECT DISTINCT pm.meta_value FROM {$wpdb->postmeta} pm
			LEFT JOIN {$wpdb->posts} p ON p.ID = pm.post_id
			WHERE pm.meta_key = '%s' 
			AND p.post_status = 'publish'
			ORDER BY pm.meta_value", 
			$key
		) 
	);

	return $result;
}