Parameter to find all records or exclude NULL - tsql

Question

I have found several articles on how to accomplish the reverse of what I want to do with several methods (IS NULL, CASE, COALESCE), but I think at this point I am more confused than ever after reading all this and probably making solution harder than it needs to be. I am new to T-SQL and I am currently using VS 2005 to build a basic medical report.
I have the Date Range parameter working properly by using a convert command to ignore time stamp giving me all records for the day or date range. I am now wanting to filter SSRS reprot by perliminary report date to find records with preliminary report, or, all records in table.
The database has NULL if no preliminary report was created
The database has time stamp if preliminary report was created. (showing date and time it was created)
I need to find all records not NULL, or all records. (using a parameter)
I have a parameter "Display Prelim Reports only?" #PrelimOnly with a YES or NO answer.
If I use the following it will show all records correctly (all records not NULL showing only records with Prelim report/time stamp present)
LIS_Results.Prelim_Report_Date <> '#PrelimOnly' ----User selects YES it passes NULL
however, if user selects NO, how would I get it to display all records including NULL?
Thank you for any help
Thank you both for your help, it was ultimately a combination of both that got it going. Syntax is as follows.
WHERE (#PrelimOnly = 'NO') AND (CONVERT(VARCHAR(10), LIS_Results.Final_Report_Date, 101) BETWEEN #ReportStartDate AND #ReportEndDate) OR (LIS_Results.Prelim_Report_Date IS NOT NULL) AND (CONVERT(VARCHAR(10), LIS_Results.Final_Report_Date, 101) BETWEEN #ReportStartDate AND #ReportEndDate)

Answer 1

Use an if statement in tsql to say if parameter is yes select records from table where conditions are true.
If no select records from table where conditions are true and date field is not null.

Answer 2

Since #PrelimOnly can only be YES or NO, use:
SELECT
...
FROM
...
WHERE #PrelimOnly = 'NO' or LIS_Results.Prelim_Report_Date is not null
...
If the parameter is NO, the left hand condition of the OR is satisfied and all rows are returned, otherwise only those non-null rows will be returned, as required.

Answer 3

here exist small trick:
((LIS_Results.Prelim_Report_Date <> '#PrelimOnly') OR (1=#AllowNull))
if user selected NO - set AllowNull argument to 1, other way set it to 0
NOTE: AllowNull - it is custom additional argument, you should add the same way as #PrelimOnly
another possible approach:
((LIS_Results.Prelim_Report_Date <> '#PrelimOnly') OR ('NO'='#PrelimOnly'))
for you full query you should do like this:
WHERE
(CONVERT(VARCHAR(10), LIS_Results.Final_Report_Date, 101) BETWEEN ReportStartDate AND ReportEndDate) AND
(
LIS_Results.Prelim_Report_Date is not null
or
('#PrelimOnly' = 'NO') // if instead of NO VS sends empty string replace it here
)

Answer 4

It was a combination of Iiya and Ian that got me the solution however the syntax was not complete and is as follows.
WHERE (#PrelimOnly = 'NO') AND (CONVERT(VARCHAR(10), LIS_Results.Final_Report_Date, 101)
BETWEEN #ReportStartDate AND #ReportEndDate) OR (LIS_Results.Prelim_Report_Date IS NOT NULL)
AND (CONVERT(VARCHAR(10), LIS_Results.Final_Report_Date, 101) BETWEEN #ReportStartDate AND
#ReportEndDate)
It requeired the Date and Time paramater to be repeated so that both paramters would still work, and the #PrelimOnly = 'NO' had to be first.