title: MySQL sum only new rows
body: <p>I have a MySQL table with this format</p>
  
  <pre><code>transaction |  user   | amount | week
  ------------|---------|--------|------
       1      | user_1  |  100   |  1
       2      | user_2  |   50   |  1
       3      | user_1  |   50   |  2
       4      | user_3  |  200   |  2
  </code></pre>
  
  <p>I know how to calculate the sum amount for each week in MySQL, but is there a way in MySQL to calculate new users sum amount per week?</p>
  
  <p>So for this table it would be:</p>
  
  <pre><code>week 1 = 150
  week 2 = 200
  </code></pre>