您所做的基本上是url_id组内的交叉连接：mr x上一个。您需要限制连接条件以获得正确的结果：

What you do there is basically cross join within url_id group: mr x prev. You need to restrict the join condition to get the correct result:

left outer join moz prev on mr.url_id = prev.url_id and prev.num = 2
where mr.num = 1