title: SQL/Hive count distinct column
body: <p>How do I do this in Hive?</p>
  
  <pre><code>    columnA       columnB    columnC
       100.10      50.60       30
       100.10      50.60       30
       100.10      50.60       20
       100.10      70.80       40
  </code></pre>
  
  <p>Output should be:</p>
  
  <pre><code>  columnA   columnB    No_of_distinct_colC
    100.10    50.60       2
    100.10    70.80       1
  </code></pre>
  
  <p>Query that I think is correct: </p>
  
  <pre><code>SELECT columnA,columnB,COUNT(distinct column C)
  from table_name
  group by columnA,columnB
  </code></pre>
  
  <p>Is this correct? SQL is fine too.</p>
  
  <p>UPDATE: How do I find the standard deviation of columnC? Need this asap.</p>