Case Statement with IS NULL not acting as required - sql

Question

Good Evening,
I have the following sql code and need to replace the NULL values within a sub query. As you can tell from the code I have tried using the ISNULL function and case where = NULL.
Could someone please help?
Select Student_Details.STU_ID ,
( Select case ISNULL( s1stu_disability_type.DISABILITY_TYPE_CD , '' )
when '' then 'NO'
else 'YES'
end
from s1stu_disability_type
Where Student_Details.STU_ID = s1stu_disability_type.STU_ID
and DISABILITY_TYPE_CD = '$HEAR'
) as 'Hearing Disability'
from S1STU_DET as Student_Details

Answer 1

Your where clause in your subquery rejects all rows but those where the column DISABILITY_TYPE_CD is '$HEAR'. Consequently, the case statement will always take the else route, as that column will never, ever be null or empty ('').
What exactly are you trying to do?
You query can better be written as
select sd.STU_ID ,
dt.DISABILITY_TYPE_CD
from S1STU_DET sd
join s1stu_disability_type dt on dt.STU_ID = sd.STU_D
and dt.DISABILITY_TYPE_CD = '$HEAR'
It's almost certian that relationship between student and student disability has a zero-to-many cardinalilty, which is to say that each student has zero or more disabilities.
As a result, your original query, with its correlated subquery, will return 1 row per student, but per the SQL standard, it's luck of the draw as to which matching disability gets selected by the subquery.
My query above will return one row per student with a matching disability. Students without a matching disability are excluded. To change that to include all students, you want to change the [inner] join to a left [outer] join. Each student will then be represented in the result set at least once. If the student has no matching disabilities, all columns for the student disability table will be 'null'.
If, as I suspect, what you're trying to do is identify students as to whether or not they have a hearing disability (or some particular type of disability), you need to summarize things. A query like this will likely do you:
select sd.STU_ID ,
case sign(coalesce(hd.cnt,0))
when 1 then 'YES'
else 'NO'
end as HAS_HEARING_DISABILITY
from S1STU_DET sd
left join ( select STU_ID ,
count(*) as cnt
from s1stu_disability
where DISABILITY_TYPE_CD = '$HEAR'
group by STU_ID
) hd on hd.STU_ID = sd.STU_ID

Answer 2

Try this:
Select Student_Details.STU_ID ,
IsNull(( Select case ISNULL( s1stu_disability_type.DISABILITY_TYPE_CD , '' )
when '' then 'NO'
else 'YES'
end
from s1stu_disability_type
Where Student_Details.STU_ID = s1stu_disability_type.STU_ID
and DISABILITY_TYPE_CD = '$HEAR'
), 'No') as 'Hearing Disability'
from S1STU_DET as Student_Details
Basically, the ISNULL function has to be outside the subquery for it to work the way you want it to. Think of it this way, if the subquery does not return any rows, the output will be null whether you have an isnull check inside the subquery or not.

Answer 3

You shouldn't need a subquery (assuming your subquery only returns one record):
Select
Student_Details.STU_ID,
case WHEN Student_Disability.DISABILITY_TYPE_CD IS NULL
THEN 'NO'
ELSE 'YES'
END
as 'Hearing Disability'
from S1STU_DET as Student_Details
LEFT JOIN s1stu_disability_type Student_Disability
ON Student_Details.STU_ID = Student_Disability.STU_ID
and Student_Disability.DISABILITY_TYPE_CD = '$HEAR'