MySQL 5.7

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CREATE TABLE `Users` (
 `user_ID` int(11) unsigned NOT NULL AUTO_INCREMENT,
 `username` varchar(25) NOT NULL,
 `last_login` datetime NOT NULL,
 `last_logout` datetime NOT NULL,
 PRIMARY KEY (`user_ID`),
 UNIQUE KEY `username` (`username`)
) ENGINE=InnoDB AUTO_INCREMENT=1813 DEFAULT CHARSET=utf8
INSERT INTO `Users` VALUES 
(1, 'User1', '2023-01-01 08:00:32', '2023-01-01 08:10:41'),
(2, 'User2', '2023-01-01 08:00:15', '2023-01-01 08:05:09'),
(3, 'User3', '2023-01-02 13:00:04', '2023-01-02 13:07:12'),
(4, 'User4', '2023-01-03 12:55:03', '2023-01-03 13:05:44'),
(5, 'User5', '2023-01-03 13:00:30', '2023-01-03 13:15:00'),
(6, 'User6', '2023-01-03 12:01:19', '2023-01-03 12:09:50');

Records: 6  Duplicates: 0  Warnings: 0


SELECT LoginDate,
       GROUP_CONCAT(username) AS 'Concurrent_users',
       COUNT(username) AS 'Total_users'
   FROM
(SELECT DATE(u1.last_login) AS LoginDate,
       u1.username,
       u1.last_login,
       u1.last_logout
FROM Users u1
JOIN Users u2
 ON u1.user_ID <> u2.user_ID
  AND u2.last_login BETWEEN u1.last_login AND u1.last_logout
UNION ALL
SELECT DATE(u2.last_login) AS LoginDate,
       u2.username,
       u2.last_login,
       u2.last_logout
FROM Users u1
JOIN Users u2
 ON u1.user_ID <> u2.user_ID
  AND u2.last_login BETWEEN u1.last_login AND u1.last_logout) a
GROUP BY LoginDate;




LoginDate
Concurrent_users
Total_users


2023-01-01
User1,User2
2

2023-01-03
User4,User5
2