title: How to check for null Json propery in Azure Stream Analytics query?
body: <p>We have an input stream of Json events from Event Hub in a following form:</p>
  
  <pre><code> ...
   { "DeviceId": null, "ReportDateUtc": "2015-05-12T20:57:13.0000000Z", ... },
   { "DeviceId": "device123", "ReportDateUtc": "2015-05-12T20:57:13.0000000Z", ... }
   ...
  </code></pre>
  
  <p>When I test-run the following query, the output record count is 0:</p>
  
  <pre><code>SELECT
      *
  FROM
      [events-input]
  WHERE DeviceId IS NULL
  </code></pre>
  
  <p>Looks like Json nulls are not exactly SQL NULLs, so what would be the proper way to check for null values in a query?</p>