CODEWITHSUNDEEP
sqlsql-server
Alex_404
Alex_404

Reputation: 409

Calculating date intervals from a daily-grained fact table

I have the data for student absence which I got after some transformations. The data is day by day:

WITH datasample AS (
    SELECT 1 AS StudentID, 20180101 AS DateID, 0 AS AbsentToday, 0 AS AbsentYesterday UNION ALL
    SELECT 1, 20180102, 1, 0 UNION ALL
    SELECT 1, 20180103, 1, 1 UNION ALL
    SELECT 1, 20180104, 1, 1 UNION ALL
    SELECT 1, 20180105, 1, 1 UNION ALL
    SELECT 1, 20180106, 0, 1 UNION ALL
    SELECT 2, 20180101, 0, 0 UNION ALL
    SELECT 2, 20180102, 1, 0 UNION ALL
    SELECT 2, 20180103, 1, 1 UNION ALL
    SELECT 2, 20180104, 0, 1 UNION ALL
    SELECT 2, 20180105, 1, 0 UNION ALL
    SELECT 2, 20180106, 1, 1 UNION ALL
    SELECT 2, 20180107, 0, 1
)
SELECT *
FROM datasample
ORDER BY StudentID, DateID

I need to add a column (AbsencePeriodInMonth) which would calculate the student's absence period during the month. For example, StudentID=1 was absent in one consecutive period during the month and StudentID=2 had two periods, something like this:

StudentID DateID    AbsentToday AbsentYesterday AbsencePeriodInMonth
1         20180101  0           0               0
1         20180102  1           0               1
1         20180103  1           1               1
1         20180104  1           1               1
1         20180105  1           1               1
1         20180106  0           1               0
2         20180101  0           0               0
2         20180102  1           0               1
2         20180103  1           1               1
2         20180104  0           1               0
2         20180105  1           0               2
2         20180106  1           1               2
2         20180107  0           1               0

My goal is actually to calculate the consecutive absent days prior to each day in the fact table, I think I can do it if I get the AbsencePeriodInMonth column, by having this added to my query after the *:

,CASE WHEN AbsentToday = 1 THEN DENSE_RANK() OVER(PARTITION BY StudentID, AbsencePeriodInMonth ORDER BY DateID)
           ELSE 0
     END AS DaysAbsent

Any idea on how I can add that AbsencePeriodInMonth or maybe calculate the consecutive absent days in some other way?

Upvotes: 1

Views: 70

Answers (2)

Gordon Linoff
Gordon Linoff

Reputation: 1269923

You can identify each period by counting the number of 0s before hand. Then you can enumerate them using dense_rank().

select ds.*,
       (case when absenttoday = 1 then dense_rank() over (partition by studentid order by grp)
             else 0
        end) as AbsencePeriodInMonth
from (select ds.*, sum(case when absenttoday = 0 then 1 else 0 end) over (partition by studentid order by dateid) as grp
      from datasample ds
     ) ds
order by StudentID, DateID;

Here is a SQL Fiddle.

Upvotes: 1

Ajay Gupta
Ajay Gupta

Reputation: 1845

Using Recursive CTE and Dense_Rank

WITH datasample AS (
    SELECT 1 AS StudentID, 20180101 AS DateID, 0 AS AbsentToday, 0 AS AbsentYesterday UNION ALL
    SELECT 1, 20180102, 1, 0 UNION ALL
    SELECT 1, 20180103, 1, 1 UNION ALL
    SELECT 1, 20180104, 1, 1 UNION ALL
    SELECT 1, 20180105, 1, 1 UNION ALL
    SELECT 1, 20180106, 0, 1 UNION ALL
    SELECT 2, 20180101, 0, 0 UNION ALL
    SELECT 2, 20180102, 1, 0 UNION ALL
    SELECT 2, 20180103, 1, 1 UNION ALL
    SELECT 2, 20180104, 0, 1 UNION ALL
    SELECT 2, 20180105, 1, 0 UNION ALL
    SELECT 2, 20180106, 1, 1 UNION ALL
    SELECT 2, 20180107, 0, 1
), cte as
(Select *,DateID as dd 
from datasample 
where AbsentToday = 1 and AbsentYesterday = 0

union all

Select d.*, c.dd 
from datasample d
join cte c
on d.StudentID = c.StudentID and d.DateID = c.DateID + 1 
    where d.AbsentToday = 1
), cte1 as
(
Select *, DENSE_RANK() over (partition by StudentId order by dd) as de 
from cte
)
Select d.*, IsNull(c.de,0) as AbsencePeriodInMonth
from cte1 c 
right join datasample d
on d.StudentID = c.StudentID and c.DateID = d.DateID
order  by d.StudentID, d.DateID

Upvotes: 1

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