I've been caught out on the QotD before by not reading the question properly, but I agree that the SQL being different to the actual quesiton seemed a little overly confusing.

However, as there's 4 records in the table, regardless of knowledge of the NULLIF function surely the output was always going to be all 4 options?

Perhaps to test knowledge of NULLIF, the question might have been better to include NULLIF as a where clause, something like:

select

id,

nullif(y,0) as 'test 1',

nullif(y,9) as 'test 2'

from

#whatif

where

nullif(y,0) is null

or nullif(y,9) is null

and then have the same 4 answers with a "Check all that apply" option?