如何这一段mysql语句转成Oracle语句

SELECT
@rownum row,
@rownum :=@rownum + 1 AS num,
date_format(
DATE_SUB(
'2017-12-31',
INTERVAL @rownum MONTH
),
'%Y-%m'
) AS all_day
FROM
(SELECT @rownum:=-1) AS r_init,
(
SELECT
c.id
FROM
t_order c
LIMIT 12
) AS c_init

https://my.oschina.net/0312/blog/393051