MySQL 8.0

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CREATE TABLE `clients` (
  `id` INT(11) NOT NULL AUTO_INCREMENT,
  `name` VARCHAR(45) NULL,
  `dt` DATE NULL,
  `company_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`));
INSERT INTO `clients` (`name`, `dt`, `company_id`) VALUES 
('user1', '2019-03-20', 1),
('user2', '2019-03-15', 1),
('user3', '2019-03-01', 2),
('user4', '2019-04-20', 3),
('user5', '2019-04-12', 1),
('user6', '2019-03-01', 1),
('user7', '2019-04-01', 2),
('user8', '2019-03-17', 2),
('user9', '2019-04-06', 3),
('user10', '2019-04-13', 1);
SELECT
  company_id,
  YEAR(dt) AS `year`,
  MONTH(dt) AS `month`,
CASE WHEN DAY(dt) < 11 THEN 1
     WHEN DAY(dt) < 18 THEN 2
     WHEN DAY(dt) < 25 THEN 3
                       ELSE 4 END AS `week_number`, 
  count(id) AS `cnt`, 
  GROUP_CONCAT(id ORDER BY id) AS `client_list`
FROM clients 
GROUP BY 1,2,3,4
ORDER BY 1,2,3,4


company_id
year
month
week_number
cnt
client_list


1
2019
3
1
1
6

1
2019
3
2
1
2

1
2019
3
3
1
1

1
2019
4
2
2
5,10

2
2019
3
1
1
3

2
2019
3
2
1
8

2
2019
4
1
1
7

3
2019
4
1
1
9

3
2019
4
3
1
4

company_id	year	month	week_number	cnt	client_list
1	2019	3	1	1	6
1	2019	3	2	1	2
1	2019	3	3	1	1
1	2019	4	2	2	5,10
2	2019	3	1	1	3
2	2019	3	2	1	8
2	2019	4	1	1	7
3	2019	4	1	1	9
3	2019	4	3	1	4