oracle 排序字段自增长

<insert id="insertGoodsDescription" parameterClass="goodsDescription" >
<selectKey resultClass="int" keyProperty="serviceDescriptionId" >
SELECT goods_description_SEQ.nextval AS service_description_id  from dual
</selectKey>
insert into product_service_description (
service_description_id,
label_name,
label_icon,
label_description,
sort_num,
product_id,
is_default,
is_delete,
ADD_TIME,
ADD_USER_ID,
EDIT_TIME,
EDIT_USER_ID)
values (
#serviceDescriptionId#,
#labelName#,
#labelIcon#,
#labelDescription#,
nvl (#sortNum#,  (
SELECT MAX(sort_num) + 1
FROM product_service_description) ),

<!--#sortNum#,-->
#productId#,
#isDefault#,
'N',
#addTime#,
#addUserId#,
#editTime#,
#editUserId#
)
</insert>

<!-- 添加微信团活动 -->

<insert id="save" parameterType="WeixinActivity">

INSERT INTO  weixin_activity
(
WEIXIN_NAME,
BANNER_IMG_URL,
START_TIME,
END_TIME ,
GROUP_PRICE,
DISCOUNT_LAB,
MINIMUM_JOIN_MEMBER_NUM,
ORDER_BY,
IS_STATUS,
BEGIN_WAIT_TIME,
ADD_USER_ID,
EDIT_TIME,
IS_DELETE,
ADD_TIME,
EDIT_USER_ID
) values
(
#{weixinName},
#{bannerImgUrl},
#{startTime},
#{endTime},
#{groupPrice},
#{discountLab},
#{minimumJoinMemberNum},
IFNULL(#{sortBy},  (
SELECT id
FROM (
SELECT MAX(order_by) + 1 AS id
FROM weixin_activity) t)),

<!--         (select id from   -->
<!--    (SELECT MAX(order_by) + 1 as id FROM weixin_activity) t), -->

#{isStatus},
#{beginWaitTime},
#{addUserId},
#{editTime},
'N',
#{addTime},
#{editUserId}
)
</insert>