T-SQL windowing functions - counting the gaps

Question

I want to count the gaps (columns filled with NULL) in a window v, but I don't know how.

IF OBJECT_ID('tempdb..#X') IS NOT NULL DROP TABLE #X;

CREATE TABLE #X
(
    ID INT IDENTITY(1,1) PRIMARY KEY,
    v INT
);

INSERT INTO #X
    SELECT 121 UNION ALL SELECT NULL UNION ALL SELECT NULL 
    UNION ALL SELECT 312 UNION ALL SELECT NULL UNION ALL SELECT NULL UNION ALL SELECT NULL
    UNION ALL SELECT 123 UNION ALL SELECT NULL UNION ALL SELECT NULL 
    UNION ALL SELECT 415 UNION ALL SELECT 416 UNION ALL SELECT NULL UNION ALL SELECT NULL UNION ALL SELECT NULL
    UNION ALL SELECT 200;


SELECT 
    ID, v, s, n, m, x, c
FROM
    (SELECT 
         ID, v, 
         MAX(v) OVER (PARTITION BY c) s,
         ROW_NUMBER() OVER (PARTITION BY c ORDER BY ID DESC) n,
         ROW_NUMBER() OVER (PARTITION BY c ORDER BY ID) - 1 m,
         COUNT(CASE WHEN v IS NULL THEN 1 END) OVER (PARTITION BY c) x,
         c
     FROM
         (SELECT 
              ID, v,
              -- c =  COUNT(CASE WHEN v IS NULL THEN 1 END) OVER (ORDER BY ID)
              c = COUNT(v) OVER (ORDER BY ID)
          FROM 
              #X) a
    ) a
ORDER BY 
    ID;

Calculation c is close, but it doesn't work on two filled columns in a row.

Could anyone give a hint please?

---

Thank you guys, this works for me now.

Answer 1

If you want to enumerate the periods of NULL-ness, you can use a cumulative count the get the non-null values and then enumerate them:

select t.*,
       (case when v is null then dense_rank() over (partition by v order by null_grp)
        end) as newcolumn
from (select t.*,
             count(v) over (order by id) as null_grp
      from t
     ) t;

Here is a db<>fiddle.

Answer 2

select *, case when v is null then count(null_grp_start) over (order by id) end null_grp_id
from   (select *,case when v is null and lag(v,1,1) over (order by id) is not null then 1 end as null_grp_start
        from   t
        ) t

-

+----+--------+----------+-------------+
| ID |   v    | null_grp | null_grp_id |
+----+--------+----------+-------------+
|  1 | 121    | (null)   | (null)      |
|  2 | (null) | 1        | 1           |
|  3 | (null) | (null)   | 1           |
|  4 | 312    | (null)   | (null)      |
|  5 | (null) | 1        | 2           |
|  6 | (null) | (null)   | 2           |
|  7 | (null) | (null)   | 2           |
|  8 | 123    | (null)   | (null)      |
|  9 | (null) | 1        | 3           |
| 10 | (null) | (null)   | 3           |
| 11 | 415    | (null)   | (null)      |
| 12 | 416    | (null)   | (null)      |
| 13 | (null) | 1        | 4           |
| 14 | (null) | (null)   | 4           |
| 15 | (null) | (null)   | 4           |
| 16 | 200    | (null)   | (null)      |
+----+--------+----------+-------------+