Row number with lag event partition

Question

I would like to insert a row number that is partitioned by the previous flag.

I have tried using the lag function within the partition, but am having challenges with NULLs.

I can determine the different block of X's by multiple date differences determined by the LAG and LEAD function and other key flags. So each 0 in the LAG and LEAD columns indicate that the rows are "linked."

My table looks like this and I want the row number to restart at the second block of X's:

Person_ID  Event_flag  Row_number  LAG_datediff  LEAD_datediff  FLAG  FLAG2
A           X          1           NULL          0              1     NULL
A           X          2           0             40             NULL  NULL
A           Y          1           40            150            NULL  NULL
A           X          1           150           0              1     NULL
A           X          2           0             NULL           NULL  1

Answer 1

You need an ordering column to number rows. It's difficult without sample data, but if your table is ordered by one date column, the next approach may help to get your expected results:

Data:

CREATE TABLE #Data (
    Event_Date date,
    Person_ID varchar(1), 
    Event_flag varchar(1)
)
INSERT INTO #Data
   (Event_date, Person_ID, Event_flag)
VALUES
    ('20190910', 'A', 'X'),
    ('20190911', 'A', 'X'),
    ('20190912', 'A', 'Y'),
    ('20190913', 'A', 'X'),
    ('20190914', 'A', 'X'),
    ('20190915', 'A', 'X'),
    ('20190916', 'A', 'Y')

Statement:

;WITH ChangesCTE AS (
    SELECT
        Event_Date,
        Person_ID,
        Event_flag,
        CASE 
            WHEN Event_flag = LAG(Event_flag) OVER (ORDER BY Event_Date) THEN 0
            ELSE 1
        END AS Change_ID
    FROM #Data
), GroupsCTE AS (
    SELECT
        Event_Date,
        Person_ID,
        Event_flag,
        SUM([Change_ID]) OVER (ORDER BY Event_Date) AS Group_ID
    FROM ChangesCTE
)
SELECT 
    Person_ID,
    Event_flag,
    ROW_NUMBER() OVER (PARTITION BY Group_ID ORDER BY Event_Date) AS Rn
FROM GroupsCTE

Output:

--------------------------
Person_ID   Event_flag  Rn
--------------------------
A           X           1
A           X           2
A           Y           1
A           X           1
A           X           2
A           X           3
A           Y           1