(select t1.goods_id  goods_id from 1dcq_goods t1 order by t1.goods_id )
UNION all

(select t2.goods_id  goods_id from 1dcq_goods t2 order by t2.goods_id desc )

如上sql语句，结果如下，可以看出是没有顺序的：

下面语句：

(select t1.goods_id  goods_id from 1dcq_goods t1 order by t1.goods_id limit 999999999 )
UNION all
(select t2.goods_id  goods_id from 1dcq_goods t2 order by t2.goods_id desc limit 999999999)

结果如下，可以看出是排序的，一个升序，一个降序，然后合并：

可见"limit"关键字不能少！