使用 lag() 以及定义组的累积和：

select t.*,
       sum(case when prev_createddate > dateadd(day, -60, createddate) then 0 else 1 end) over  (partition by customername order by createddate) as grp
from (select t.*,
             lag(createddate) over (partition by customername order by createddate) as prev_createddate
      from t
     ) t;

然后使用 row_number() 每组内：

select t.*,
       row_number() over (partition by customername, grp order by createddate) as mypartition
from (select t.*,
             sum(case when prev_createddate > dateadd(day, -60, createddate) then 0 else 1 end) over  (partition by customername order by createddate) as grp
      from (select t.*,
                   lag(createddate) over (partition by customername order by createddate) as prev_createddate
            from t
           ) t
     ) t;

请注意 partition 是一个非常糟糕的列名称，因为它是一个sql关键字。