MySQL中的Left join...on

/**
 * 獲得購買過該商品的人還買過的商品
 *
 * @access  public
 * @param   integer     $goods_id
 * @return  array
 */
function get_also_bought($goods_id)
{
    $sql = 'SELECT COUNT(b.goods_id ) AS num, g.goods_id, g.goods_name, g.goods_thumb, g.goods_img, g.shop_price, g.promote_price, g.promote_start_date, g.promote_end_date ' .
            'FROM ' . $GLOBALS['ecs']->table('order_goods') . ' AS a ' .
            'LEFT JOIN ' . $GLOBALS['ecs']->table('order_goods') . ' AS b ON b.order_id = a.order_id ' .
            'LEFT JOIN ' . $GLOBALS['ecs']->table('goods') . ' AS g ON g.goods_id = b.goods_id ' .
            "WHERE a.goods_id = '$goods_id' AND b.goods_id <> '$goods_id' AND g.is_on_sale = 1 AND g.is_alone_sale = 1 AND g.is_delete = 0 " .
            'GROUP BY b.goods_id ' .
            'ORDER BY num DESC ' .
            'LIMIT ' . $GLOBALS['_CFG']['bought_goods'];
    $res = $GLOBALS['db']->query($sql);

}