Why does not see NULL value?

Question

SQL Server

left join(
select patientname, isnull(fish, N'HI') fish from
( select PatientName,
case
when DocumentNodeValue = 'True' then N'Hi'
when DocumentNodeValue is null then N'Hi'
end FISH
from Patient join course on PatientRef = PatientID
join Document on CourseRef = CourseID
join DocumentNode on DocumentRef = DocumentID
left join CourseGroup on CourseGroupRef = CourseGroupID
where
DocumentNodeStaticGUIDRef = 'eaba2d91-b394-4b9e-8e85-0bad18bbf37d') rr) Q On a.PatientName = Q.PatientName
An attempt was made to replace NULL value through the function and through the constraint (implemented in the code above). But when executing the query, it still returns NULL.

Answer 1

Why does not see NULL value?

So it's not there.