title: How to apply sometimes a filter in a tablix on SQL Report Services, and sometimes don't?
body: <p>I am doing a report with Visual Studio, and I am newbie on this kind of reports. I have a parameter (StringID), and in its properties I allow multiple values. I also have a tablix with the following filter:</p>
  
  <p><strong>Expression</strong>: [@ExternalID] -- <strong>Operator</strong>: In -- <strong>Value</strong>: [@StringID]</p>
  
  <p>It works well, but I also want to allow the user to select all the registries (ie, not to filter by StringID), and if this field is blank it shows all the registries with ExternalID empty, and that is not what I want. So the question is:</p>
  
  <p>Can I have an optional filter or something with this functionality? </p>