title: Values in mysql resetting back to 0 after update query
body: <p>this is what my table looks like</p>
  
  <pre><code>_____________________________________________________________
  |  id  |  val1  |  val2  |  val3  |  val4  |  val5  |  val6  |
  |______|________|________|________|________|________|________|
  |   1  |  1300  |  1300  |     0  |  2000  |  1300  |     0  |
  |______|________|________|________|________|________|________|
  |  10  |     0  |     0  |     0  |  2000  |     0  |     0  |
  |______|________|________|________|________|________|________|
  </code></pre>
  
  <p>Now I update</p>
  
  <pre><code>UPDATE points set val5 = CASE WHEN id = 10 THEN val5 + 500 WHEN id = 1 THEN val5 + 500 END WHERE id IN ('10','1')
  </code></pre>
  
  <p>This is what my table looks like</p>
  
  <pre><code>_____________________________________________________________
  |  id  |  val1  |  val2  |  val3  |  val4  |  val5  |  val6  |
  |______|________|________|________|________|________|________|
  |   1  |  1800  |  1800  |     0  |  2000  |  1800  |     0  |
  |______|________|________|________|________|________|________|
  |  10  |   500  |   500  |     0  |  2000  |   500  |     0  |
  |______|________|________|________|________|________|________|
  | 157  |     0  |     0  |     0  |   800  |     0  |     0  |
  |______|________|________|________|________|________|________|
  </code></pre>
  
  <p>Now I update again <strong>(this is where it's broken)</strong></p>
  
  <pre><code>UPDATE points set val5 = CASE WHEN id = 1 THEN val5 + 500 END, val6 = CASE WHEN id = 10 THEN val6 + 500 END WHERE id IN ('10','1')
  </code></pre>
  
  <p>My table looks like this</p>
  
  <pre><code>_____________________________________________________________
  |  id  |  val1  |  val2  |  val3  |  val4  |  val5  |  val6  |
  |______|________|________|________|________|________|________|
  |   1  |  2300  |  2300  |     0  |  2000  |  2300  |     0  |
  |______|________|________|________|________|________|________|
  |  10  |  1000  |     0  |   500  |  2000  |     0  |   500  |
  |______|________|________|________|________|________|________|
  | 157  |     0  |     0  |     0  |   800  |     0  |     0  |
  |______|________|________|________|________|________|________|
  | 158  |     0  |     0  |     0  |   800  |     0  |     0  |
  |______|________|________|________|________|________|________|
  </code></pre>
  
  <p>Did you notice how val2 and val5 went back to zero even though I didn't tell it to? what am I missing?</p>