MySQL 8.0

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CREATE TABLE `events` (
`EventID`  int NOT NULL ,
`EventTypeID`  int NULL ,
`StartDateTime`  datetime NULL ,
`EndDateTime`  datetime NULL ,
PRIMARY KEY (`EventID`)
)
;
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('11139', '670', '2023-03-28 08:30:00', '2023-03-29 08:30:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('10937', '1', '2023-03-21 08:00:00', '2023-03-21 14:00:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('11161', '713', '2023-03-29 08:30:00', '2023-03-30 08:30:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('10938', '64', '2023-03-21 08:00:00', '2023-03-21 09:00:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('10969', '61', '2023-03-22 08:00:00', '2023-03-22 14:00:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('10967', '27', '2023-03-23 09:00:00', '2023-03-23 15:00:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('11093', '126', '2023-03-27 10:00:00', '2023-03-27 19:00:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('11094', '710', '2023-03-27 10:00:00', '2023-03-27 18:00:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('10970', '3', '2023-04-12 08:30:00', '2023-04-12 23:30:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('10939', '1', '2023-04-26 08:30:00', '2023-04-26 14:30:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('10972', '60', '2023-04-12 08:30:00', '2023-04-12 14:30:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('10973', '61', '2023-04-14 08:00:00', '2023-04-14 14:00:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('10917', '721', '2023-04-01 08:30:00', '2023-04-02 08:30:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('11180', '21', '2023-04-15 10:00:00', '2023-04-15 18:30:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('10953', '53', '2023-04-05 09:00:00', '2023-04-05 21:00:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('11186', '673', '2023-04-19 10:00:00', '2023-04-19 15:00:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('10954', '64', '2023-04-05 09:00:00', '2023-04-05 10:00:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('11091', '126', '2023-04-22 10:00:00', '2023-04-22 19:00:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('10901', '670', '2023-04-11 08:30:00', '2023-04-12 08:30:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('11092', '710', '2023-04-22 10:00:00', '2023-04-22 18:00:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('11130', '721', '2023-05-20 08:30:00', '2023-05-22 12:00:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('11151', '670', '2023-05-21 08:00:00', '2023-05-22 08:00:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('10941', '53', '2023-05-04 09:00:00', '2023-05-04 21:00:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('10995', '27', '2023-05-23 09:00:00', '2023-05-23 15:00:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('10942', '64', '2023-05-04 09:00:00', '2023-05-04 10:00:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('10945', '1', '2023-05-26 08:00:00', '2023-05-26 14:00:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('10977', '61', '2023-05-06 08:00:00', '2023-05-06 14:00:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('10997', '3', '2023-05-30 08:00:00', '2023-05-30 23:00:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('10892', '25', '2023-05-16 08:30:00', '2023-05-16 14:30:00');
INSERT INTO `events` (`EventID`, `EventTypeID`, `StartDateTime`, `EndDateTime`) VALUES ('10999', '60', '2023-05-30 08:00:00', '2023-05-30 14:00:00');
select 
  EventTypeID
  ,mo
  ,group_concat(d order by d separator ', ')
from (
  
SELECT
  EventTypeID,
  month(StartDateTime) mo,
  day(StartDateTime) d
FROM
    `events`
WHERE
    `events`.StartDateTime > now()
) t
Group by EventTypeID, mo
order by mo


EventTypeID
mo
group_concat(d order by d separator ', ')


670
3
28

1
3
21

126
3
27

713
3
29

27
3
23

64
3
21

710
3
27

61
3
22

60
4
12

126
4
22

670
4
11

64
4
5

61
4
14

673
4
19

53
4
5

710
4
22

21
4
15

721
4
1

3
4
12

1
4
26

721
5
20

670
5
21

64
5
4

61
5
6

60
5
30

53
5
4

27
5
23

25
5
16

3
5
30

1
5
26
EventTypeID	mo	group_concat(d order by d separator ', ')
670	3	28
1	3	21
126	3	27
713	3	29
27	3	23
64	3	21
710	3	27
61	3	22
60	4	12
126	4	22
670	4	11
64	4	5
61	4	14
673	4	19
53	4	5
710	4	22
21	4	15
721	4	1
3	4	12
1	4	26
721	5	20
670	5	21
64	5	4
61	5	6
60	5	30
53	5	4
27	5	23
25	5	16
3	5	30
1	5	26
EventTypeID	mo	group_concat(d order by d separator ', ')
670	3	28
1	3	21
126	3	27
713	3	29
27	3	23
64	3	21
710	3	27
61	3	22
60	4	12
126	4	22
670	4	11
64	4	5
61	4	14
673	4	19
53	4	5
710	4	22
21	4	15
721	4	1
3	4	12
1	4	26
721	5	20
670	5	21
64	5	4
61	5	6
60	5	30
53	5	4
27	5	23
25	5	16
3	5	30
1	5	26