title: Sparksql filtering (selecting with where clause) with multiple conditions
body: <p>Hi I have the following issue:</p>
  
  <pre><code>numeric.registerTempTable("numeric"). 
  </code></pre>
  
  <p>All the values that I want to filter on are literal null strings and not N/A or Null values.  </p>
  
  <p>I tried these three options:</p>
  
  <ol>
  <li><p><code>numeric_filtered = numeric.filter(numeric['LOW'] != 'null').filter(numeric['HIGH'] != 'null').filter(numeric['NORMAL'] != 'null')</code></p></li>
  <li><p><code>numeric_filtered = numeric.filter(numeric['LOW'] != 'null' AND numeric['HIGH'] != 'null' AND numeric['NORMAL'] != 'null')</code></p></li>
  <li><p><code>sqlContext.sql("SELECT * from numeric WHERE LOW != 'null' AND HIGH != 'null' AND NORMAL != 'null'")</code></p></li>
  </ol>
  
  <p>Unfortunately, numeric_filtered is always empty. I checked and numeric has data that should be filtered based on these conditions. </p>
  
  <p>Here are some sample values:</p>
  
  <p>Low          High          Normal</p>
  
  <p>3.5           5.0           null</p>
  
  <p>2.0           14.0          null</p>
  
  <p>null          38.0          null</p>
  
  <p>null          null          null</p>
  
  <p>1.0           null          4.0</p>