title: how to find colums having missing data in sparklyr
body: <p>example sample data</p>
  
  <pre><code>Si      K       Ca      Ba  Fe  Type
  71.78   0.06    8.75    0   0   1
  72.73   0.48    7.83    0   0   1
  72.99   0.39    7.78    0   0   1
  72.61   0.57    na  0   0   na
  73.08   0.55    8.07    0   0   1
  72.97   0.64    8.07    0   na  1
  73.09   na  8.17    0   0   1
  73.24   0.57    8.24    0   0   1
  72.08   0.56    8.3 0   0   1
  72.99   0.57    8.4 0   0.11    1
  na  0.67    8.09    0   0.24    1
  </code></pre>
  
  <p>we can load data into <code>sparklyr</code> with the following code</p>
  
  <pre><code>sdf_copy_to(sc,sampledata)
  </code></pre>
  
  <p>I am looking for a query that returns the columns having NA values
  for example like</p>
  
  <pre><code>si k ca fe
  1  1  1 2
  </code></pre>