title: MongoDB equivalent for an SQL query
body: <p>Any idea how to represent the following SQL condition for MongoDB</p>
  
  <pre><code>WHERE
       a = 1
  AND  b = 2
  AND  (c &gt;= 3 OR c IS NULL)
  AND  d = 4
  </code></pre>
  
  <p>Tried this, but seems not working:</p>
  
  <pre><code>{ a:1, b:2, c:{ $in:[ { $gte:3 }, { $exists: false } ] } , d:4 }
  </code></pre>
  
  <p>This doesn't work since the key 'c' gets overridden:</p>
  
  <pre><code>{ a:1, b:3, $or:[ { c:{ $gte:3 } }, { c:{ $exists:false } } ] , d:4 }
  </code></pre>
  
  <p>Any help is greatly appreciated</p>