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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');
WITH cte AS (
SELECT *, CASE WHEN Event_flag = LAG(Event_flag) OVER (PARTITION BY Person_ID ORDER BY Event_date)
THEN 0 ELSE 1 END AS flag
FROM Data
)
SELECT Event_date, Person_ID, Event_flag,
SUM(flag) OVER (ORDER BY Event_date) AS Rn
FROM cte
ORDER BY Event_date;
