title: Filter dataset on either value OR nothing
body: <p>I have a dataset that contains a lot of records. I would like to include all those records where PType in @Ptype OR PType = ''</p>
  
  <pre><code>YEARMONTH --|-- PTYPE --|-- VALUE
  201207 --|-- bike --|-- 600
  201208 --|-- moped --|-- 0
  201209 --|-- '' --|-- 0
  201210 --|-- bike --|-- 600
  201211 --|-- '' --|-- 0
  201212 --|-- car--|-- 1200
  </code></pre>
  
  <p>This is filtered using a parameter. The filtering occurs in the report on the reference to the shared dataset; </p>
  
  <pre><code>Expression:PType
  Operator: in 
  Value: @Ptype 
  </code></pre>
  
  <p>Now when @PType contains ('bike','moped')</p>
  
  <p><strong>I get this resultset:</strong></p>
  
  <pre><code>YEARMONTH --|-- PTYPE --|-- VALUE
  201207 --|-- bike --|-- 600
  201208 --|-- moped --|-- 0
  201210 --|-- bike --|-- 600
  </code></pre>
  
  <p><strong>but I would like</strong> </p>
  
  <pre><code>YEARMONTH --|-- PTYPE --|-- VALUE
  201207 --|-- bike --|-- 600
  201208 --|-- moped --|-- 0
  201209 --|-- '' --|-- 0
  201210 --|-- bike --|-- 600
  201211 --|-- '' --|-- 0
  </code></pre>
  
  <p>So I would also like to see those records where Ptype is Empty/NULL/Nothing.
  Is this possible?</p>
  
  <p>If so, how?</p>