Invert a Boolean expression which can return UNKNOWN
You could enclose the condition in a CASE expression that returns a binary result, for instance 1 or 0:

SELECT
  ...
FROM
  ...
WHERE
  CASE WHEN someColumn = someValue THEN 1 ELSE 0 END = 1
;


Negating the expression will give you all the other rows from the same data source, including those where someColumn is null:

SELECT
  ...
FROM
  ...
WHERE
  NOT CASE WHEN someColumn = someValue THEN 1 ELSE 0 END = 1
  -- or: CASE WHEN someColumn = someValue THEN 1 ELSE 0 END &lt;&gt; 1
;


Since SQL Server 2012 you also have the IIF function, which is just a wrapper around a binary CASE like above. So, this CASE expression:

CASE WHEN someColumn = someValue THEN 1 ELSE 0 END


will look like this if rewritten using IIF:

IIF(someColumn = someValue, 1, 0)


And you can use it exactly the same way as the CASE expression. There will be no difference in performance, only the code will be slightly more concise, possibly cleaner that way too.