(select distinct(temp_2.colA) from temp_1 inner join temp_2 on temp_1.colC = temp_2.colA) t3 ...

insert/*trace*/ into t1 values(1,1), (1, 2), (null, 3), (2, 4), (3,5); insert/*trace*/ into t2 values(1, 1), (3, 3), (5,5), (6, null); ...