title: Sum duplicate row values
body: <p>I'm trying to sum the values of rows two of which have duplicate values, the table I have is below: </p>
  
  <p>Table name (Customers)</p>
  
  <pre><code>value   years   total
  1           30    30
  3           10    10
  4           15    15
  4           25    25
  </code></pre>
  
  <p>I would ideally like to finally have: </p>
  
  <pre><code>value  years   total
  1      30      30
  3      10      10
  4      40      40
  </code></pre>
  
  <p>I've tried using <code>SELECT DISTINCT</code> and <code>GROUP BY</code> to get rid of the duplicate row, also the join in the code below isn't necessary. Regardless both commands come to no avail. Here's my code too: </p>
  
  <pre><code>SELECT DISTINCT 
    value,
    years,
    SUM(customer.years) AS total 
  FROM customer 
  INNER JOIN language 
    ON customer.expert=language.l_id 
  GROUP BY 
    expert, 
    years;
  </code></pre>
  
  <p>But that produces a copy of the first table, any input welcome. Thanks!!!</p>