我要用SQL算出客户的购买间隔，要用SQL如何实现呢？，参考过前面有人写过的，但是逻辑不对。还请各位帮忙看看。谢谢。
我参考写的：
select
a.card_num,DATEDIFF(a.day,b.day) as interval
from
(select
card_num,day,
row_number() over(partition by (card_num) order by day asc) as num
from
test) a
left outer join
(select
card_num,day,
row_number() over(partition by (card_num) order by day asc) as num
from
test) b
on a.card_num=b.card_num
where a.card_num=1 and b.card_num=2