title: Query to use GROUP BY multiple columns
body: <p>I have a table full of patients/responsible parties/insurance carrier combinations (e.g. patient Jim Doe's responsible party is parent John Doe who has insurance carrier Aetna Insurance). For each of these combinations, they have a contract that has multiple payments. For this particular table, I need to write a query to find any parent/RP/carrier combo that has multiple contract dates in the same month. Is there anyway to do this?</p>
  
  <p>Example table:</p>
  
  <pre><code>ContPat | ContResp | ContIns | ContDue
  ------------------------------------------------------
  53      | 13       | 27      | 2012-01-01 00:00:00.000
  53      | 13       | 27      | 2012-02-01 00:00:00.000
  53      | 15       | 27      | 2012-03-01 00:00:00.000
  12      | 15       | 3       | 2011-05-01 00:00:00.000
  12      | 15       | 3       | 2011-05-01 00:00:00.000
  12      | 15       | 3       | 2011-06-01 00:00:00.000
  12      | 15       | 3       | 2011-07-01 00:00:00.000
  12      | 15       | 3       | 2011-08-01 00:00:00.000
  12      | 15       | 3       | 2011-09-01 00:00:00.000
  </code></pre>
  
  <p>In this example, I would like to generate a list of all the duplicate months for any Patient/RP/Carrier combinations. The 12/15/3 combination would be the only row returned here, but I'm working with thousands of combinations. </p>
  
  <p>Not sure if this is possible using a <code>GROUP BY</code> or similar functions. Thanks in advance for any advice!</p>