Simple Oracle's Merge statement[example]

step 1: Create two tables [EXAM and REEXAM] and insert                   values

insert all into exam(ROLLNO,SCORE) VALUES(111,80)

               into exam(ROLLNO,SCORE) VALUES(112,null)

               into exam(ROLLNO,SCORE) VALUES(113,68)

               into exam(ROLLNO,SCORE) VALUES(114,72)

               into exam(ROLLNO,SCORE) VALUES(115,null)

               into exam(ROLLNO,SCORE) VALUES(116,89)

               into exam(ROLLNO,SCORE) VALUES(117,76)

               into exam(ROLLNO,SCORE) VALUES(118,null)

               into exam(ROLLNO,SCORE) VALUES(119,null)

select * from dual;

EXAM table: 

ROLLNO	SCORE
111	80
112	(Null)
113	68
114	72
115	(Null)
116	89
117	76
118	(Null)
119	(Null)

INSERT all INTO REEXAM(ROLLNO,SCORE)VALUES(112,77)

                    INTO REEXAM(ROLLNO,SCORE)VALUES(115,67)

                    INTO REEXAM(ROLLNO,SCORE)VALUES(118,58)

                    INTO REEXAM(ROLLNO,SCORE)VALUES(119,83)

SELECT * FROM dual;

REEXAM table:

ROLLNO	SCORE
112	76
115	94
118	82
119	58

step 2: Merge tables

 MERGE INTO exam a USING (SELECT ROLLNO,score FROM reexam) b on (a.rollno=b.rollno)

  when MATCHED then UPDATE set a.score=b.score;

ROLLNO	SCORE
111	80
112	76
113	68
114	72
115	94
116	89
117	76
118	82
119	58

4 rows merged.