Hi @lanfengye,
the operator operates with semantics similar to SQL (see previous topic here), where Row2 “column1” could be an “a” in reality – it might just be missing in the dataset so the operator filters it out. The DB Filter node would do so, too, but the old Row Filter did not.
For the next feature release AP 5.5 (due in Summer) we added a variant of “does not equal” that retains missing values to make it easier to use in complex filters:
The existing operator will be relabeled to “Is not equal (nor missing)” to make it clear what it does and the new operator will be called “Is not equal”, which should make it more intuitive for users of the old Row Filter.