join update and nvl

I have two tables:nncreate table t1 (a_id number, b_id number, col varchar2(1));ncreate table t2 (a_id number, b_id number, col2 varchar2(1));nncreate unique index t2_uidx on t2 (a_id, b_id);nninsert into t1 values (1, null, 'a');ninsert into...

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