MySQL 8.0

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CREATE TABLE users(id INTEGER PRIMARY KEY, reg_date DATE);
INSERT INTO users VALUES (1, '2019-01-02'), (2, '2019-01-02'), (3, '2019-01-02'), (4, '2019-02-02'), (5, '2019-02-02'), (6, '2019-03-02');
SELECT max(rn) AS count, reg_date
FROM (SELECT row_number() OVER (ORDER BY reg_date) AS rn
           , reg_date
      FROM users) AS x
GROUP BY reg_date
ORDER BY reg_date;


count
reg_date


3
2019-01-02

5
2019-02-02

6
2019-03-02