Oracle 11g Release 2

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CREATE TABLE t1
    ("ID" int, "DT" timestamp, "VALUE" int)
;











INSERT ALL 
    INTO t1 ("ID", "DT", "VALUE")
         VALUES (1, '01-Feb-2019 12:00:00 AM', 100)
    INTO t1 ("ID", "DT", "VALUE")
         VALUES (2, '01-Jan-2019 12:00:00 AM', 101)
    INTO t1 ("ID", "DT", "VALUE")
         VALUES (2, '01-Feb-2019 12:00:00 AM', 102)
SELECT * FROM dual
;
3 rows affected

CREATE TABLE t2
    ("ID" int, "DT" timestamp, "VALUE" int)
;
INSERT ALL 
    INTO t2 ("ID", "DT", "VALUE")
         VALUES (1, '01-Jan-2019 12:00:00 AM', 101)
    INTO t2 ("ID", "DT", "VALUE")
         VALUES (1, '01-Feb-2019 12:00:00 AM', 100)
    INTO t2 ("ID", "DT", "VALUE")
         VALUES (1, '01-Mar-2019 12:00:00 AM', 100)
    INTO t2 ("ID", "DT", "VALUE")
         VALUES (2, '01-Jan-2019 12:00:00 AM', 101)
    INTO t2 ("ID", "DT", "VALUE")
         VALUES (2, '01-Feb-2019 12:00:00 AM', 102)
    INTO t2 ("ID", "DT", "VALUE")
         VALUES (2, '01-Jan-2019 12:00:00 AM', 101)
    INTO t2 ("ID", "DT", "VALUE")
         VALUES (2, '01-Feb-2019 12:00:00 AM', 102)
    INTO t2 ("ID", "DT", "VALUE")
         VALUES (2, '01-Mar-2019 12:00:00 AM', 100)
SELECT * FROM dual
;
8 rows affected

  select  t2.id,t2.dt,t2.value from t2  LEFT JOIN t1 ON
    to_char(t1.dt,'yyyymm') = to_char(t2.dt,'yyyymm')
    and t1.id = t2.id
  ORDER BY id,dt


ID
DT
VALUE


1
01-JAN-19 12.00.00.000000 AM
101

1
01-FEB-19 12.00.00.000000 AM
100

1
01-MAR-19 12.00.00.000000 AM
100

2
01-JAN-19 12.00.00.000000 AM
101

2
01-JAN-19 12.00.00.000000 AM
101

2
01-FEB-19 12.00.00.000000 AM
102

2
01-FEB-19 12.00.00.000000 AM
102

2
01-MAR-19 12.00.00.000000 AM
100


select DISTINCT t2.id,t2.dt,t2.value from t2  LEFT JOIN t1 ON
    to_char(t1.dt,'yyyymm') = to_char(t2.dt,'yyyymm')
    and t1.id = t2.id
  ORDER BY id,dt


ID
DT
VALUE


1
01-JAN-19 12.00.00.000000 AM
101

1
01-FEB-19 12.00.00.000000 AM
100

1
01-MAR-19 12.00.00.000000 AM
100

2
01-JAN-19 12.00.00.000000 AM
101

2
01-FEB-19 12.00.00.000000 AM
102

2
01-MAR-19 12.00.00.000000 AM
100


    select  t2.id,t2.dt,coalesce(t1.value,t2.value) as value 
    from t2  LEFT JOIN t1 ON
        to_char(t1.dt,'yyyymm') = to_char(t2.dt,'yyyymm') and
        t1.id = t2.id
      ORDER BY id,dt


ID
DT
VALUE


1
01-JAN-19 12.00.00.000000 AM
101

1
01-FEB-19 12.00.00.000000 AM
100

1
01-MAR-19 12.00.00.000000 AM
100

2
01-JAN-19 12.00.00.000000 AM
101

2
01-JAN-19 12.00.00.000000 AM
101

2
01-FEB-19 12.00.00.000000 AM
102

2
01-FEB-19 12.00.00.000000 AM
102

2
01-MAR-19 12.00.00.000000 AM
100

ID	DT	VALUE
1	01-JAN-19 12.00.00.000000 AM	101
1	01-FEB-19 12.00.00.000000 AM	100
1	01-MAR-19 12.00.00.000000 AM	100
2	01-JAN-19 12.00.00.000000 AM	101
2	01-JAN-19 12.00.00.000000 AM	101
2	01-FEB-19 12.00.00.000000 AM	102
2	01-FEB-19 12.00.00.000000 AM	102
2	01-MAR-19 12.00.00.000000 AM	100