ROWNUM C0         M               SS
---------- ---------- ---------- ----------
         1 a          a__               3.5
         2 a          abd               1.5
         3 a          abe               3.5
         4 a          ace               5.5
         5 b          a__                35
         6 b          abd                15
         7 b          abe                35
         8 b          ace                55

ROWNUM       M        a          b
----------  --------- ---------- ----------
         1     a__       3.5        35
         2     abd       1.5        15
         3     abe       3.5        35
         4     ace       5.5        55

CREATE TABLE T4 (
C0 VARCHAR2(10),
C1 VARCHAR2(10),
C2 NUMBER
);
INSERT INTO T4  VALUES ('a','abd',1);
INSERT INTO T4  VALUES ('a','abd',2);
INSERT INTO T4  VALUES ('a','abe',3);
INSERT INTO T4  VALUES ('a','abe',4);
INSERT INTO T4  VALUES ('a','ace',5);
INSERT INTO T4  VALUES ('a','ace',6);

INSERT INTO T4  VALUES ('b','abd',10);
INSERT INTO T4  VALUES ('b','abd',20);
INSERT INTO T4  VALUES ('b','abe',30);
INSERT INTO T4  VALUES ('b','abe',40);
INSERT INTO T4  VALUES ('b','ace',50);
INSERT INTO T4  VALUES ('b','ace',60);



SELECT ROWNUM,rr.C0,rr.M, rr.ss
FROM
(
SELECT C0,C1 M, AVG(C2) ss FROM T4  GROUP BY C0, C1
UNION
SELECT C0,SUBSTR(C1,1,1)||'__'  ,AVG(C2) ss FROM T4  GROUP BY C0,SUBSTR(C1,1,1) ) rr
ORDER BY rr.C0,rr.M ASC;