The answer will be 0 because
a<> a --> false
a<>null--> null (no row is returned)
----------------------------------
b<>a--->true
b<>null--->null(no row is returned)
----------------------------------
c<>a--->true
c<>null--->null(no row is returned)
---------------------------------
<>null” is UNKNOWN (neither true or false), no row is returned because every condition must be true in order for the AND operator to return a TRUE result.
A good read:
http://www.sqlbadpractices.com/using-not-in-operator-with-null-values/
