SQL Server 2019

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CREATE TABLE mytable(
   CaseID     INTEGER  NOT NULL 
  ,UserID     VARCHAR(2) NOT NULL
  ,OpenedOn   DATE  NOT NULL
  ,ClosedDate DATE 
  ,Value      INTEGER  NOT NULL
);
INSERT INTO mytable(CaseID,UserID,OpenedOn,ClosedDate,Value) VALUES (1,'U1','2020-01-04','2020-02-05',500);
INSERT INTO mytable(CaseID,UserID,OpenedOn,ClosedDate,Value) VALUES (2,'U1','2020-01-08',NULL,120);
INSERT INTO mytable(CaseID,UserID,OpenedOn,ClosedDate,Value) VALUES (3,'U3','2020-01-16',NULL,350);
INSERT INTO mytable(CaseID,UserID,OpenedOn,ClosedDate,Value) VALUES (4,'U2','2020-01-21','2020-01-25',100);
INSERT INTO mytable(CaseID,UserID,OpenedOn,ClosedDate,Value) VALUES (5,'U1','2020-01-25','2020-04-08',150);
INSERT INTO mytable(CaseID,UserID,OpenedOn,ClosedDate,Value) VALUES (6,'U2','2020-02-05',NULL,790);
INSERT INTO mytable(CaseID,UserID,OpenedOn,ClosedDate,Value) VALUES (7,'U4','2020-02-14','2020-03-18',190);
INSERT INTO mytable(CaseID,UserID,OpenedOn,ClosedDate,Value) VALUES (8,'U4','2020-02-19',NULL,490);
INSERT INTO mytable(CaseID,UserID,OpenedOn,ClosedDate,Value) VALUES (9,'U1','2020-02-21','2020-02-27',140);
INSERT INTO mytable(CaseID,UserID,OpenedOn,ClosedDate,Value) VALUES (10,'U2','2020-03-07','2020-03-17',640);
INSERT INTO mytable(CaseID,UserID,OpenedOn,ClosedDate,Value) VALUES (11,'U1','2020-03-17','2020-05-12',420);
INSERT INTO mytable(CaseID,UserID,OpenedOn,ClosedDate,Value) VALUES (12,'U3','2020-03-22',NULL,810);
INSERT INTO mytable(CaseID,UserID,OpenedOn,ClosedDate,Value) VALUES (13,'U2','2020-04-05','2020-05-19',320);
INSERT INTO mytable(CaseID,UserID,OpenedOn,ClosedDate,Value) VALUES (14,'U1','2020-04-18',NULL,180);
INSERT INTO mytable(CaseID,UserID,OpenedOn,ClosedDate,Value) VALUES (15,'U4','2020-04-16','2020-05-22',210);
INSERT INTO mytable(CaseID,UserID,OpenedOn,ClosedDate,Value) VALUES (16,'U3','2020-05-19',NULL,230);

16 rows affected

userID	year_month	no_cases_still_open	value_open	no_closed	value_closed
U1	2020-01-01	3	770	0	0
U1	2020-02-01	2	270	2	640
U1	2020-03-01	3	690	0	0
U1	2020-04-01	3	720	1	150
U1	2020-05-01	2	300	1	420
U2	2020-01-01	0	0	1	100
U2	2020-02-01	1	790	0	0
U2	2020-03-01	1	790	1	640
U2	2020-04-01	2	1110	0	0
U2	2020-05-01	1	790	1	320
U3	2020-01-01	1	350	0	0
U3	2020-02-01	1	350	0	0
U3	2020-03-01	2	1160	0	0
U3	2020-04-01	2	1160	0	0
U3	2020-05-01	3	1390	0	0
U4	2020-01-01	0	0	0	0
U4	2020-02-01	2	680	0	0
U4	2020-03-01	1	490	1	190
U4	2020-04-01	2	700	0	0
U4	2020-05-01	1	490	1	210

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