Oracle 11g Release 2

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CREATE TABLE MYTABLE (A VARCHAR(20), B VARCHAR(20))
INSERT INTO MYTABLE VALUES ('A1', 'B1')
1 rows affected

INSERT INTO MYTABLE VALUES ('A2', 'B2')
1 rows affected

INSERT INTO MYTABLE VALUES ('A3', 'B3')
1 rows affected

SELECT CASE WHEN T1.A = LEAD(T1.A) OVER (ORDER BY T1.A)  THEN T1.A
            ELSE T1.B
       END Merged
FROM MYTABLE T1
LEFT JOIN MYTABLE T2 ON T1.A <> T2.A;


MERGED


A1

B1

A2

B2

A3

B3