title: how can i build sql query?
body: <p>I have table with date, the change name, the value of the change in cargo in stock like this:</p>
  
  <pre><code>Date|Change_name|Change_val|
  
  2015.02.13 add 20
  
  2015.02.13 del 5
  
  2015.03.01 add 17
  
  2015.03.01 add 2
  
  2015.03.02 del 7
  </code></pre>
  
  <p>I want to get the table, which include day balance like this:</p>
  
  <pre><code>2015.02.13 15
  
  2015.03.01 34
  
  2015.03.02 27
  </code></pre>
  
  <p>How can i get this with SQL query?</p>