MySQL 8.0

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CREATE TABLE user_private_message (
  id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
  user_from_id INT NOT NULL,
  send_time TIMESTAMP NOT NULL DEFAULT NOW()
);
INSERT INTO user_private_message(user_from_id, send_time)
VALUES
  (1, '2007-01-01 00:00:00'),
  (1, '2007-01-01 00:00:00'),
  (1, '2007-01-02 00:00:00'),
  (1, '2007-01-02 00:00:00'),
  (2, '2007-01-02 00:00:00'),
  (1, '2007-01-03 00:00:00'),
  (2, '2007-01-03 00:00:00'),
  (3, '2007-01-03 00:00:00'),
  (1, '2007-01-04 00:00:00'),
  (1, '2007-01-04 00:00:00'),
  (1, '2007-01-04 00:00:00'),
  (1, '2007-01-05 00:00:00'),
  (2, '2007-01-05 00:00:00');

Records: 13  Duplicates: 0  Warnings: 0


SELECT 
  DATE(send_time), 
  COUNT(user_from_id) AS message_count, 
  COUNT(DISTINCT user_from_id) AS senders_count
FROM user_private_message
GROUP BY DATE(send_time);


DATE(send_time)
message_count
senders_count


2007-01-01
2
1

2007-01-02
3
2

2007-01-03
3
3

2007-01-04
3
1

2007-01-05
2
2


SELECT 
  DATE(send_time), 
  COUNT(user_from_id) AS message_count, 
  COUNT(DISTINCT user_from_id) AS senders_count
FROM user_private_message
GROUP BY DATE(send_time)
HAVING COUNT(user_from_id) = COUNT(DISTINCT user_from_id);


DATE(send_time)
message_count
senders_count


2007-01-03
3
3

2007-01-05
2
2

DATE(send_time)	message_count	senders_count
2007-01-01	2	1
2007-01-02	3	2
2007-01-03	3	3
2007-01-04	3	1
2007-01-05	2	2