title: a simple way to sum a result from UNION in MySql
body: <p>I have a union of three tables (t1,t2,t3). 
  Each rerun exactly the same number of records, first column is id, second amount:  </p>
  
  <pre><code>1  10
  2  20
  3  20
  
  1  30
  2  30
  3  10
  
  1  20
  2  40
  3  50
  </code></pre>
  
  <p>Is there a <strong>simple</strong> in sql way to sum it up to only get:</p>
  
  <pre><code>1   60
  2   80
  3   80
  </code></pre>