Requête avec left join

$querystr2 = "
SELECT pm1.post_id AS post_id, pm1.meta_value AS meta_value1, pm2.meta_value AS meta_value2, pm3.meta_value AS meta_value3, pm4.meta_value AS meta_value4
	FROM wp_postmeta pm1
	LEFT JOIN $wpdb->postmeta pm2
	ON pm1.post_id = pm2.post_id 
	LEFT JOIN $wpdb->postmeta pm3
	ON pm2.post_id = pm3.post_id 
	LEFT JOIN $wpdb->postmeta pm4
	ON pm3.post_id = pm4.post_id
	WHERE pm1.post_id= $post_idval 
	AND pm1.meta_key = 'nomprogrammes' 
	AND pm2.meta_key = 'urlsite' 
	AND pm3.meta_key = 'category'
	AND pm4.meta_key = 'nomprogramme_taxo' 
	";
	$result2=mysql_query($querystr2) or die (mysql_error());
	while ($var2 = mysql_fetch_array($result2) ) {
}