SQL & PL/SQL

when the unique constraint of the second table includes three columns?

I have two tables A and B and I want to update a column in table A using an update of an inline view of the two tables.

Table A:
col1 primary key
col2

Table B:
col1 u1
col2 u2
col3 u3
col4
col5

Table A has col1 as primary key, table B has a unique constraint over col1, col2 and col3.

Table A and table B are joined in this way:

select  A.col1, A.col2 as KEY_OLD, B.col5 as KEY_NEW
from    A, B
where (B.col1 = 'literal_1') and (B.col2 = 'literal_2') and (A.col1 = B.col3) and (A.col2 = B.col4)

And now I want to update this view and set KEY_OLD = KEY_NEW.

Because of the first three conditions in the where-clause for every entry in table A there is just one matching entry in table B.
So the table A should be key-preserved in the join.

But when making the update

update (select  A.col1, A.col2 as KEY_OLD, B.col5 as KEY_NEW
            from    A, B
            where (B.col1 = 'literal_1') and (B.col2 = 'literal_2') and (A.col1 = B.col3) and (A.col2 = B.col4)
           )
set     KEY_OLD = KEY_NEW;

I get an error ORA-01779.

I have made updates on inline views in this way many times in the last weeks and it work very good.
But until now the unique contstraint on table B had just one column;-)

So what I need is to get it working with the unique constraint on table B applying to three columns.

Many thanks in advance for any help.

Regards Hartmut