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The best translation for the value NULL in SQL I can work with is unknown. Therefor, the test if null = null then is false, because we cannot know if the unknown thing on the left side of the = is the same unknown thing on the right side.
However, sometimes, it seems reasonable that null = null should indeed be true. In such a case, sys_op_map_nonnull can be used.
Warning: sys_op_map_nonnull is undocumented and there is no guarantee that it is always working as described here.
select a, b, c from t where
sys_op_map_nonnull(f) = sys_op_map_nonnull(g);