MySQL 5.5

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CREATE tABLE users (userid int,regdate date,idprovince int )
INSERT INTO users VALUEs (1,'2020-01-21',1),(2,'2020-02-21',1),(3,'2020-03-21',1),
(4,'2020-01-21',2),(5,'2020-02-21',2),(6,'2020-03-21',2)
    SELECT 
        regmonth, 
        IF(@idprovince = idprovince,@num:=@num + `num` , @num:= `num`)  as num,
        @idprovince := idprovince as idprovince
    FROM
        (SELECT 
            DATE_FORMAT(regdate, '%Y-%m-01') AS regmonth,
                idprovince,
                COUNT(userid) AS num
        FROM
            users
        GROUP BY DATE_FORMAT(regdate, '%Y-%m-01') , idprovince
        ORDER BY idprovince , DATE_FORMAT(regdate, '%Y-%m-01')) t1,(SELECT @num:=0,@idprovince := 0) t2


regmonth
num
idprovince


2020-01-01
1
1

2020-02-01
2
1

2020-03-01
3
1

2020-01-01
1
2

2020-02-01
2
2

2020-03-01
3
2

regmonth	num	idprovince
2020-01-01	1	1
2020-02-01	2	1
2020-03-01	3	1
2020-01-01	1	2
2020-02-01	2	2
2020-03-01	3	2