How use NULL value SSRS parameter to retrieve rows containing NULLS - reporting-services

Question

I've read many posts concerning NULLS and SSRS report parameters, but none of them appear to solve my problem.
I have a user-selectable SSRS report parameter #NoteType which can be 1, 3, 4 or NULL. The database table I'm working with (eventText) has a column NoteValue containing 1 of those 4 values. Unfortunately, I can't alter the table design to substitute an integer for that NULL value.
My report works fine if #NoteType = 1, 3 or 4; but not if #NoteType = NULL. If #NoteType = NULL then, depending on my paramaterized WHERE clause (discussed below) my report has either no rows, or it has all of the rows for which NoteValue = 1, 3 or 4. I can't figure out how to retrieve the rows for which NoteValue = NULL if the user selects #NoteType = NULL.
I've configured the #NoteType parameter to allow NULL values, with Note Value as the parameter's value field and Note Type as the label field. The parameter values are obtained from a UNION query:
SELECT ett.NoteValue AS [Note Value],
ett.NoteDescription AS [Note Type]
FROM eventTextType ett
UNION
SELECT NULL AS [Note Value],
'Blank' AS [Note Type]
ORDER BY [Note Type]
The eventTextType table has only 3 rows in which the NoteValue column value is 1, 3 or 4 respectively. There is no row with NoteValue = NULL in the eventTextType table, but there are many such rows in the eventText table. That's why I'm using a UNION query to include NULL as one of the #NoteType parameter values.
The UNION query's output is:
Note Value Note Type
4 Annuity
1 Balance
NULL Blank
3 Division
I suspect that my problem is in the WHERE clause which uses the #NoteType parameter to retrieve rows for the report. Here's what I have tried:
WHERE et.NoteValue = #NoteType -- works fine for #NoteType = 1, 3 or 4; but no rows are returned if #NoteType = NULL
WHERE et.NoteValue = IIF(#NoteType IS NULL, NULL, #NoteType) -- same result
WHERE et.NoteValue = CASE WHEN #NoteType IS NULL THEN NULL ELSE #NoteType END -- same result (expected, since this is equivalent to the IFF version)
WHERE et.NoteValue = COALESCE(#NoteType, et.NoteValue) -- works fine for #NoteType = 1, 3 or 4; but if #NoteType = NULL returns all rows with NoteValue column value 1, 3 or 4 but no rows with NoteValue column value = NULL
What WHERE clause should I be using?

Answer 1

Assuming that your dataset for your main dataset is a dataset query (not a stored proc) then I would simply add a valid parameter value (I've used 0 in this exmaple) to the parameter list and substitute that in the dataset query.
So, change the dataset query that provide parameter values to
SELECT ett.NoteValue AS [Note Value],
ett.NoteDescription AS [Note Type]
FROM eventTextType ett
UNION
SELECT 0 AS [Note Value],
'Blank' AS [Note Type]
ORDER BY [Note Type]
Now in you main dataset query you can change your where clause to this (which will also handle multi-value parameters if that's required)
WHERE ISNULL(et.NoteValue, 0) IN(#NoteType)
Unless I've missed something obvious, that should be it.