TSQL, counting coherent days of holiday - tsql

Question

I hope someone can help me on this one. :-)
I wish to count coherent periods of holiday to see if anyone had coherent holiday more than three days in a row. In other words it is not enough to count the number of days overall. The days have to be coherent. In the example of my data below I have illustrated three people with each their own days of holiday. Person 1234 has two periods of two days of holiday in a row, so this person has no periods above three days since there is a day in between two periods (the 3rd). Person 1235 and 1236 each have one period above three days. Time of day in the timestamps has no relevance, so data can be formatted as just date.
What I have:
ID
Start
1234
2022-01-01 00:00:00
1234
2022-01-02 00:00:00
1234
2022-01-04 06:50:00
1234
2022-01-05 06:50:00
1235
2022-01-04 06:50:00
1235
2022-01-05 06:50:00
1235
2022-01-06 00:00:00
1236
2022-01-01 00:00:00
1236
2022-01-02 00:00:00
1236
2022-01-03 06:50:00
1236
2022-01-04 06:50:00
1236
2022-01-05 06:50:00
1236
2022-01-08 00:00:00
What I hope to get:
ID
N holidays > 3 days
1234
0
1235
1
1236
1
Anyways, any help will be appreciated!
Kind regards,
Jacob

Answer 1

This is a "gaps and islands" problem. You need to first group the data into "islands", which in your case is groups of consecutive holidays. Then summarize them in your final result set
Side note: your question requests greater than 3 days, but your expected output uses greater than or equal to 3 so I used that instead.
DROP TABLE IF EXISTS #Holiday;
DROP TABLE IF EXISTS #ConsecutiveHoliday
CREATE TABLE #Holiday (ID INT,StartDateTime DATETIME)
INSERT INTO #Holiday
VALUES (1234,'2022-01-01 00:00:00')
,(1234,'2022-01-02 00:00:00')
,(1234,'2022-01-04 06:50:00')
,(1234,'2022-01-05 06:50:00')
,(1235,'2022-01-04 06:50:00')
,(1235,'2022-01-05 06:50:00')
,(1235,'2022-01-06 00:00:00')
,(1236,'2022-01-01 00:00:00')
,(1236,'2022-01-02 00:00:00')
,(1236,'2022-01-03 06:50:00')
,(1236,'2022-01-04 06:50:00')
,(1236,'2022-01-05 06:50:00')
,(1236,'2022-01-08 00:00:00');
WITH cte_Previous AS (
SELECT A.ID,B.StartDate
,IsHolidayConsecutive = CASE WHEN DATEADD(day,-1,StartDate) /*Current day minus 1*/ = LAG(StartDate) OVER (PARTITION BY ID ORDER BY StartDate) /*Previous holiday date*/
THEN 0
ELSE 1
END
FROM #Holiday AS A
CROSS APPLY (SELECT StartDate = CAST(StartDateTime AS DATE)) AS B
),
cte_Groups AS (
SELECT *,GroupID = SUM(IsHolidayConsecutive) OVER (PARTITION BY ID ORDER BY StartDate)
FROM cte_Previous
)
/*Groups of holidays taken consecutively*/
SELECT ID
,StartDate = MIN(StartDate)
,EndDate = MAX(StartDate)
,NumOfDays = COUNT(*)
INTO #ConsecutiveHoliday
FROM cte_Groups
GROUP BY ID,GroupID
ORDER BY ID,StartDate
/*See list of consecutive holidays taken*/
SELECT *
FROM #ConsecutiveHoliday
/*Formatted result*/
SELECT ID
,[N holidays >= 3 days] = COUNT(CASE WHEN NumOfDays >= 3 THEN 1 END)
FROM #ConsecutiveHoliday
GROUP BY ID