title: Sum values from multiple rows into one row
body: <p>In SQL Server 2012, I have a  table <code>my_table</code> that has columns <code>state, month, ID</code>, and <code>sales</code>. </p>
  
  <p>My goal is to merge different rows that have the same <code>state, month, ID</code> into one row while summing the <code>sales</code> column of these selected rows into the merged row.</p>
  
  <p>For example: </p>
  
  <pre><code>state    month    ID    sales
  -------------------------------
  FL       June     0001   12,000
  FL       June     0001    6,000
  FL       June     0001    3,000
  FL       July     0001    6,000
  FL       July     0001    4,000  
  TX       January  0050    1,000
  MI       April    0032    5,000
  MI       April    0032    8,000
  CA       April    0032    2,000
  </code></pre>
  
  <p>This what I am supposed to get</p>
  
  <pre><code>state    month    ID    sales
  -------------------------------
  FL       June     0001   21,000
  FL       July     0001   10,000  
  TX       January  0050    1,000
  MI       April    0032   13,000
  CA       April    0032    2,000
  </code></pre>
  
  <p>I did some research, and I found that the self join is supposed to do something similar to what I am supposed to get.</p>