Counting consecutive duplicate field with SQL

Question

I have this data in myTable :

  Date           Status    PersonID
-----------------------------------------
   2018/01/01         2        2015     ┐  2
   2018/01/02         2        2015     ┘
   2018/01/05         2        2015     ┐
   2018/01/06         2        2015       3
   2018/01/07         2        2015     ┘
   2018/01/11         2        2015     - 1
   2018/01/01         2        1018     - 1
   2018/01/03         2        1018     - 1
   2018/01/05         2        1018     ┐ 2
   2018/01/06         2        1018     ┘
   2018/01/08         2        1018     ┐ 2
   2018/01/09         2        1018     ┘
   2018/01/03         2        1625     ┐
   2018/01/04         2        1625       4
   2018/01/05         2        1625     
   2018/01/06         2        1625     ┘
   2018/01/17         2        1625     - 1
   2018/01/29         2        1625     - 1
-----------------------------------

and I need to count consecutive duplicate values like this:

This is the result I need:

   count    personid
    -----------------
    2        2015
    3        2015
    1        2015
    1        1018
    1        1018
    2        1018
    2        1018
    4        1625
    1        1625
    1        1625

I am using SQL Server 2016 - please help

Answer 1

This is a 'Gaps and Islands' problem, you can try like following.

;with cte 
     as (select *, 
                dateadd(day, -row_number() 
                                over (partition by status, personid 
                                  order by [date] ), [date]) AS grp 
         FROM   @table
     )
     ,cte1 
     AS (select *,row_number() over(partition by  personid, grp,status order by [date]) rn,
                count(*) over(partition by personid, grp) ct 
         from   cte
        ) 

select  ct as count, personid 
from cte1 
where rn=1

Online Demo

Note: You might not get the rows in same sequence as you don't have any column which can be used for ordering the way you showed in the desired output.

Answer 2

This type of problem is known as 'Gaps and Islands'. You are either identifying consecutive data sets (Islands) or range of values between two islands (Gaps). There are many different ways to achieve the results that also performs well with large data sets. You can refer the below well written articles for that.

https://www.itprotoday.com/sql-server/solving-gaps-and-islands-enhanced-window-functions

https://www.red-gate.com/simple-talk/sql/t-sql-programming/the-sql-of-gaps-and-islands-in-sequences/

https://www.sqlshack.com/data-boundaries-finding-gaps-islands-and-more/

Here is an attempt to your question.

CREATE TABLE #test 
(
     dt DATETIME
    ,Status INT
    ,PersonID INT
)

INSERT INTO #Test (dt, Status, PersonID) VALUES
('2018/01/01', 2, 2015),
('2018/01/02', 2, 2015),
('2018/01/05', 2, 2015),
('2018/01/06', 2, 2015),
('2018/01/07', 2, 2015),
('2018/01/11', 2, 2015),
('2018/01/01', 2, 1018),
('2018/01/03', 2, 1018),
('2018/01/05', 2, 1018),
('2018/01/06', 2, 1018),
('2018/01/08', 2, 1018),
('2018/01/09', 2, 1018),
('2018/01/03', 2, 1625),
('2018/01/04', 2, 1625),
('2018/01/05', 2, 1625),
('2018/01/06', 2, 1625),
('2018/01/17', 2, 1625),
('2018/01/29', 2, 1625)

;with cte_dt_from
AS
(
    SELECT PersonID, MIN(Dt) dt_from_start
    FROM #Test
    GROUP BY PersonID
),
cte_offset_num
AS
(
SELECT      T1.PersonID, T1.dt, DATEDIFF(DAY, T2.dt_from_start, T1.dt) dt_offset
FROM        #test T1
INNER JOIN  cte_dt_from T2 ON T2.PersonID = T1.PersonID
),
cte_starting_point
AS
(
    SELECT A.PersonID, A.dt_offset, ROW_NUMBER() OVER(PARTITION BY A.PersonID ORDER BY A.dt_offset) AS rownum
    FROM cte_offset_num AS A
    WHERE NOT EXISTS (
        SELECT *
        FROM cte_offset_num AS B
        WHERE B.PersonID = A.PersonID AND B.dt_offset = A.dt_offset - 1)
)
,
cte_ending_point
AS
(
    SELECT A.PersonID, A.dt_offset, ROW_NUMBER() OVER(PARTITION BY A.PersonID ORDER BY A.dt_offset) AS rownum
    FROM cte_offset_num AS A
    WHERE NOT EXISTS (
        SELECT *
        FROM cte_offset_num AS B
        WHERE B.PersonID = A.PersonID AND B.dt_offset = A.dt_offset + 1)
)
SELECT (E.dt_offset - S.dt_offset)  + 1 AS [count], S.PersonID
FROM cte_starting_point AS S
JOIN cte_ending_point AS E ON E.PersonID = S.PersonID AND E.rownum = S.rownum
ORDER BY S.PersonID;

DROP TABLE #Test;