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as null = null then it gets messy

Where (NOT (f1 is null) and (f2 is null))
or ((f1 is null) and (f2 is not null))
or ((f1 is not null) and (f2 is null))
or ((f1 is not null) and (f2 is not null) and (f1 f2))

Another way is to use the isnull function, it's a bit naughty depends on how whatever uses the data treats nulls.
for instance if nulls in an integer field are treated as zeros in the application then

isNull(f1,0) isnull(f2,0) would do the job, if you want to say the NULL = 0 !

you could use another value instead of zero e.g. -1 given that - 1 should never be in the column.

HtHs