T-SQL and SSRS multi parameter - sql-server

Question

I have a question about querying SQL from SSRS multi-value parameter.
My multi-value parameter is basically empty and depending on user-entered value so it is empty by default:
I try to make SQL query to query all data if no value is provided to the SSRS parameter and query only the data of the parameter if it is provided.
I tried these and none works so far:
SerialNumber IN (IIF(#SNList='', SerialNumber, #SNList)
SerialNumber IN (IIF(#SNList IS NULL, SerialNumber, #SNList)
SerialNumber IN (#SNList)
#1 works as I wanted to return all if the parameter is empty & if only one value is entered for the parameter but return an error if I entered more than 1 value (well, because when there are more than 1 value, we cannot use "=")
#2 will not return anything if no value is entered for parameter & if only one value is entered for the parameter but return an error if I entered more than 1 value.
#3 Works for multi/single values parameter
Is there a way to check number of values for the parameter or is there another way to make this works?

Answer 1

Multi-select parameters cannot be null, and to apply multi-select parameters SSRS will transform your query which would break if you tried to examine the variable in TSQL.
What you can do is introduce a second TSQL parameter that you set based on whether you should apply the main parameter. So your multi-select parameter #p1 is a string with a default value of "All", dataset query would be something like
if #p1IsNull = 1
begin
select *
from Sales.SalesOrderHeader
end
else
begin
select *
from Sales.SalesOrderHeader
where SalesOrderId in (#p1)
end
And #piIsNull is set with an expression
of the form
=iif( Parameters!p1.Value(0) = "All", 1, 0)
Depending on the parameter values, the SQL query will look like this:
exec sp_executesql N'if #p1IsNull = 1
begin
select *
from Sales.SalesOrderHeader
end
else
begin
select *
from Sales.SalesOrderHeader
where SalesOrderId in (N''43660'',N''43667'')
end',N'#p1IsNull int',#p1IsNull=0
or like this
exec sp_executesql N'if #p1IsNull = 1
begin
select *
from Sales.SalesOrderHeader
end
else
begin
select *
from Sales.SalesOrderHeader
where SalesOrderId in (#p1)
end',N'#p1IsNull int,#p1 nvarchar(3)',#p1IsNull=1,#p1=N'All'