Ranking by date in SQL but ignoring dates <3 days together

Question

I would like to rank some data by date to obtain a sequential list (1, 2, 3) but ignore dates that are <3 days together.

For example:

11/10/2015 = 1
11/11/2015 = 2
11/12/2015 = NULL (within 3 days of previous date)
11/15/2015 = 3

What is an efficient way of doing this please? Thank you in advance.

Answer 1

I think you're going for something like this. Knowing the RDBMS would help.

with date_lags as
    (
        select
            date,
            lag(date) over(order by date) as last_date
        from
            table
    )
select
    date,
    rank() over(order by date) as the_rank
from
    date_lags
where
    datediff(day, last_date, date) >= 3;

Answer 2

This is a bit challenging, because you don't have lag() or cumulative sum. But, you can implement it using outer apply (or a subquery):

select e.*,
       (case when CountFlag = 1
             then row_number() over (partition by CountFlag order by e.date)
        end) as newcol
from example e outer apply
     (select top 1 (case when e.date >= dateadd(day, 3, e2.date) then 1 else 0 end) as CountFlag
      from example e2
      where e2.date < e.date
      order by e2.date desc
     ) eprev;

This uses a logic trick to get the NULL value. Notice there is no WHERE , so all values are assigned a rank. The outer query uses the same column for the case condition and for the partition by clause.