MySQL 8.0

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select version();


version()


8.0.30


CREATE TABLE mytable(
   ID                   INTEGER 
  ,Username             VARCHAR(10)
  ,RegistrationDatetime VARCHAR(22)
);
INSERT INTO mytable(ID,Username,RegistrationDatetime) VALUES (1,'A','2022-01-03 12:00:00');
INSERT INTO mytable(ID,Username,RegistrationDatetime) VALUES (2,'B','2022-01-03 14:00:00');
INSERT INTO mytable(ID,Username,RegistrationDatetime) VALUES (3,'C','2022-01-04 23:00:00');
INSERT INTO mytable(ID,Username,RegistrationDatetime) VALUES (4,'D','2022-01-04 14:00:00');
INSERT INTO mytable(ID,Username,RegistrationDatetime) VALUES (5,'E','2022-01-05 14:00:00');
SELECT 
   date(RegistrationDatetime ),
   sum(count(*)) over (order by date(RegistrationDatetime ))
FROM
   mytable
GROUP BY
   date(RegistrationDatetime );


date(RegistrationDatetime )
sum(count(*)) over (order by date(RegistrationDatetime ))


2022-01-03
2

2022-01-04
4

2022-01-05
5