SQL Server 2019

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CREATE TABLE Events ( Id INT,
                      CreationDate DATE,
                      DeletionDate DATE,
                      PostId INT );
                      
INSERT INTO Events VALUES 
(1,  '2020-01-01', '2020-01-04', 1),
(2,  '2020-01-03', '2020-01-07', 2),
(3,  '2020-01-07', '2020-01-11', 1),
(4,  '2020-01-05', '2020-01-10', 2),
(5,  '2020-01-12', '2020-01-14', 1),
(6,  '2020-01-16', '2020-01-27', 2),
(7,  '2020-01-11', '2020-01-13', 1),
(8,  '2020-01-09', '2020-01-13', 2),
(9,  '2020-01-22', '2020-01-25', 1),
(10, '2020-01-24', '2020-01-28', 2),
(11, '2020-01-13', '2020-01-15', 1),
(12, '2020-01-07', '2020-01-11', 2),
(13, '2020-01-02', '2020-01-05', 1),
(14, '2020-01-10', '2020-01-13', 2);

SELECT * FROM Events ORDER BY 4,2,3;


Id
CreationDate
DeletionDate
PostId


1
2020-01-01
2020-01-04
1

13
2020-01-02
2020-01-05
1

3
2020-01-07
2020-01-11
1

7
2020-01-11
2020-01-13
1

5
2020-01-12
2020-01-14
1

11
2020-01-13
2020-01-15
1

9
2020-01-22
2020-01-25
1

2
2020-01-03
2020-01-07
2

4
2020-01-05
2020-01-10
2

12
2020-01-07
2020-01-11
2

8
2020-01-09
2020-01-13
2

14
2020-01-10
2020-01-13
2

6
2020-01-16
2020-01-27
2

10
2020-01-24
2020-01-28
2


-- https://ru.stackoverflow.com/a/1222576/229437
SELECT 
  PostId,
  MAX(Cnt) MaxCnt
FROM 
( 
  SELECT 
    e1.PostId,
    (COUNT(IIF(e2.CreationDate BETWEEN e1.CreationDate AND e1.DeletionDate, 1, NULL)) + 1) Cnt
  FROM Events e1
    LEFT JOIN Events e2 ON e2.PostId = e1.PostId AND e2.Id != e1.Id
  GROUP BY
    e1.PostId, e1.CreationDate, e1.DeletionDate
) AS X
GROUP BY 
  PostId


PostId
MaxCnt


1
3

2
4


Warning: Null value is eliminated by an aggregate or other SET operation.


-- https://ru.stackoverflow.com/a/1222582/229437
WITH cte AS ( SELECT t1.PostId, 
                     COUNT(t2.PostId) cnt, 
                     ROW_NUMBER() OVER (PARTITION BY t1.PostId ORDER BY COUNT(t2.PostId) DESC) rn
              FROM Events t1
              JOIN Events t2 ON t1.PostId = t2.PostId
                            AND t2.CreationDate BETWEEN t1.CreationDate AND t1.DeletionDate
              GROUP BY t1.PostId, t1.CreationDate )
SELECT PostId, cnt MaxCnt
FROM cte           
WHERE rn = 1


PostId
MaxCnt


1
3

2
4

Id	CreationDate	DeletionDate	PostId
1	2020-01-01	2020-01-04	1
13	2020-01-02	2020-01-05	1
3	2020-01-07	2020-01-11	1
7	2020-01-11	2020-01-13	1
5	2020-01-12	2020-01-14	1
11	2020-01-13	2020-01-15	1
9	2020-01-22	2020-01-25	1
2	2020-01-03	2020-01-07	2
4	2020-01-05	2020-01-10	2
12	2020-01-07	2020-01-11	2
8	2020-01-09	2020-01-13	2
14	2020-01-10	2020-01-13	2
6	2020-01-16	2020-01-27	2
10	2020-01-24	2020-01-28	2