ZF_DB：如何对两个或多个表进行简单选择而无需联接？

SELECT `cat`.`id_catalog`, COUNT(parent.id_catalog) - 1) AS `level` FROM `tbl_catalog` AS `cat`, `tbl_catalog` AS `parent` WHERE (cat.`left` BETWEEN parent.`left` AND parent.`right`) GROUP BY `cat`.`id_catalog` ORDER BY `cat`.`left` ASC