MySQL 5.6

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select version();


version()


5.6.51


CREATE TABLE IF NOT EXISTS `eventLog` (
  `userid` varchar(10) NOT NULL,
  `ds` date NOT NULL
);
INSERT INTO `eventLog` (`userid`, `ds`) VALUES
  ('user1', '2022-01-01'),
  ('user2', '2022-02-11'),
  ('user2', '2022-03-21'),
  ('user3', '2022-01-11'),
  ('user3', '2022-02-27'),
  ('user3', '2022-04-06');

Records: 6  Duplicates: 0  Warnings: 0


SELECT * FROM eventLog;


userid
ds


user1
2022-01-01

user2
2022-02-11

user2
2022-03-21

user3
2022-01-11

user3
2022-02-27

user3
2022-04-06


SELECT 
  DATE_FORMAT(ds, '%Y-%m-01') as ds_month,
  CONCAT(FLOOR(IF(DATE_FORMAT(ds, '%Y-%m-01') = DATE_FORMAT(ds, '%Y-01-01'),100,(COUNT(DISTINCT userid)*100.0)/(SELECT COUNT(DISTINCT userid) FROM eventLog))),'%') AS user_count
FROM eventLog
GROUP BY DATE_FORMAT(ds, '%Y-%m-01')


ds_month
user_count


2022-01-01
100%

2022-02-01
66%

2022-03-01
33%

2022-04-01
33%

userid	ds
user1	2022-01-01
user2	2022-02-11
user2	2022-03-21
user3	2022-01-11
user3	2022-02-27
user3	2022-04-06