Oracle 18c

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ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
CREATE TABLE tbl_a (id number, ndx1 number, ndx2 number, val number);
CREATE TABLE tbl_b (id number, ndx1 number, ndx2 number, val number);
BEGIN
INSERT INTO tbl_a VALUES (100, 1, 1, 2000);
INSERT INTO tbl_a VALUES (200, 1, 1, 4000);
INSERT INTO tbl_a VALUES (300, 1, 1, 6000);
INSERT INTO tbl_a VALUES (400, 1, 1, 8000);
INSERT INTO tbl_b VALUES (100, 1, 1, null);
INSERT INTO tbl_b VALUES (200, 1, 1, 10000);
INSERT INTO tbl_b VALUES (300, 1, 2, 40000);
END;
/
1 rows affected

SELECT COALESCE(b.id, a.id) AS id,
       COALESCE(b.ndx1, a.ndx1) AS ndx1,
       COALESCE(b.ndx2, a.ndx2) AS ndx2,
       COALESCE(b.val, a.val) AS val
FROM   tbl_a a
       FULL OUTER JOIN tbl_b b
       ON     a.id   = b.id
          AND a.ndx1 = b.ndx1
          AND a.ndx2 = b.ndx2
ORDER BY id, ndx1, ndx2


ID
NDX1
NDX2
VAL


100
1
1
2000

200
1
1
10000

300
1
1
6000

300
1
2
40000

400
1
1
8000