Oracle 18c

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CREATE TABLE mytable(
   Name  VARCHAR(1) NOT NULL
  ,dt  DATE  NOT NULL
  ,cnt INTEGER  NOT NULL
);

begin
INSERT INTO mytable(Name,dt,cnt) VALUES ('P',date '2020-01-01',4);
INSERT INTO mytable(Name,dt,cnt) VALUES ('P',date '2020-03-01',3);
INSERT INTO mytable(Name,dt,cnt) VALUES ('P',date '2020-05-01',2);
INSERT INTO mytable(Name,dt,cnt) VALUES ('P',date '2020-06-01',5);
INSERT INTO mytable(Name,dt,cnt) VALUES ('T',date '2019-12-01',4);
INSERT INTO mytable(Name,dt,cnt) VALUES ('T',date '2020-01-01',1);
end;
/
1 rows affected

with cte (dt, max_dt) as (
    select min(dt), max(dt) from mytable
    union all
    select dt + interval '1' month, max_dt from cte where dt < max_dt
)
select c.dt, n.name, t.cnt
from cte c
cross join (select distinct name from mytable) n
left join mytable t on t.name = n.name and t.dt = c.dt
order by n.name, c.dt


DT
NAME
CNT


01-DEC-19
P
null

01-JAN-20
P
4

01-FEB-20
P
null

01-MAR-20
P
3

01-APR-20
P
null

01-MAY-20
P
2

01-JUN-20
P
5

01-DEC-19
T
4

01-JAN-20
T
1

01-FEB-20
T
null

01-MAR-20
T
null

01-APR-20
T
null

01-MAY-20
T
null

01-JUN-20
T
null

DT	NAME	CNT
01-DEC-19	P	null
01-JAN-20	P	4
01-FEB-20	P	null
01-MAR-20	P	3
01-APR-20	P	null
01-MAY-20	P	2
01-JUN-20	P	5
01-DEC-19	T	4
01-JAN-20	T	1
01-FEB-20	T	null
01-MAR-20	T	null
01-APR-20	T	null
01-MAY-20	T	null
01-JUN-20	T	null