Null in where statement (RDB import)

I would like to filter an RDB table during import by using the where statement and check for null value of a field (pseudo code!):

'name': table = "name"
   where (
      .'field A' > // some value
      and .'field B' == // null

How to use the value for null here?

You should be able to write your null filter like this:

.'field B' is null