If your question was more than just theoretical, another way you could get the NOT 'a' values including NULLs would be to create a temporary column. I have no idea if this would perform better than just using 'OR IS NULL' in the WHERE clause based on this trivial example. I tried these two options against some real data and the query execution plans were the same except for a 'Compute Scalar' operation (due to the ISNULL operator) with an Estimated Operator Cost of 0%. It does seem in my experience though that using 'IS NULL' in a Where clause makes it difficult or impossible to get Index Seeks.

CREATE TABLE #a (id INT,nm VARCHAR(200))

INSERT INTO #a

SELECT

1

,'a'

INSERT INTO #a

SELECT

1

,NULL

SELECT

id

,nm

FROM

(

SELECT

id

,nm

,ISNULL(nm,'xxx') AS nm2 --> 'xxx' = some always unique value or use NEWID()

FROM

#a

) AS temp

WHERE

nm2 NOT IN ('a')

DROP TABLE #a