How can I identify groups of consecutive dates in SQL?

Question

Im trying to write a function which identifies groups of dates, and measures the size of the group.

I\'ve been doing this procedurally in Python until now but I\'d like

Answer 1

You can do this with a clever application of window functions. Consider the following:

select name, date, row_number() over (partition by name order by date)
from t

This adds a row number, which in your example would simply be 1, 2, 3, 4, 5. Now, take the difference from the date, and you have a constant value for the group.

select name, date,
       dateadd(d, - row_number() over (partition by name order by date), date) as val
from t

Finally, you want the number of groups in sequence. I would also add a group identifier (for instance, to distinguish between the last two).

select name, date,
       count(*) over (partition by name, val) as NumInSeq,
       dense_rank() over (partition by name order by val) as SeqID
from (select name, date,
             dateadd(d, - row_number() over (partition by name order by date), date) as val
      from t
     ) t

Somehow, I missed the part about weekdays and holidays. This solution does not solve that problem.