Question

i have task which takes a parameter and has three modes of results

Example
Parameter name is sizePram (1 = only null , 2 = only not null, 3 = both or all)

declare @sizeParam int
set @sizeParam = 1 or 2 or 3

select * from SalesLT.Product where Size =
CASE @sizeParam
    when 1 then is null
    when 2 then is not null
    when 3 then all
end

Answer 1

select * 
from SalesLT.Product 
where 
    (@sizeParam = 1 and Size is null) or
    (@sizeParam = 2 and Size is not null) or
    (@sizeParam = 3)