Need help on Update query

Question

Hi,
I have 2 tables A and B with below data and description, i want to update Table B when C1, C2 are equal and C3 is not equal by comparing with Table A. If A contains 2 records with same such condition then update any 1 record's different description in Table B.
I can achieve this by using below query but i don't want to query Table A twice. Also if this is update is possible using Cursor is also fine for me..
Please help me on this...Thanks in Advance
Query
Update B
set B.c3 = (select c3 from A
where A.c1 = B.c1
and A.c2 = B.c2
and A.c3 <> B.c3
and ROWNUM = 1)
where exists (select 1 from A
where A.c1 = B.c1
and A.c2 = B.c2
and A.c3 <> B.c3);
Table A Table B
C1 C2 C3 C1 C2 C3
1 B desc1 1 B desc3
1 B desc2 2 x desc2
2 x desc1 3 y desc3
3 y desc3
Expected Output
Table B
C1 C2 C3
1 B desc1 or desc2 (any 1 different description should get updated)
2 x desc1
3 y desc3

Answer 1

I am a bit confused. this also works without numbers in the C3 column.
drop table tablea;
drop table tableb;
create table tablea as
(select 1 C1,'B' C2, 'descA' C3 from dual union
select 1, 'B', 'descB' from dual union
select 2, 'x', 'descC' from dual union
select 3, 'y', 'descC' from dual
create table tableb as
(select 1 C1,'B' C2, 'descC' C3 from dual union
select 2, 'x', 'descB' from dual union
select 3, 'y', 'descC' from dual
merge into tableb t1
using (
           select tablea.C1, tablea.C2, max(tablea.C3) C3
           from tablea, tableb
           where tablea.c1 = tableb.c1
           and     tablea.c2 = tableb.c2
           and     tablea.c3 != tableb.c3
           group by tablea.C1, tablea.C2) t2
  on (t1.C1 = t2.C1 and t1.C2 = t2.C2 )
when matched  then
update
set T1.C3 = t2.C3;
select * from tableb;
C1     C2     C3
1     B     descB
2     x     descC
3     y     descC