title: Mysql sum of rows as row value
body: <p>I've stucked with some mysql problem with sum. I need a table of invoices like this:</p>
  
  <pre><code>id invoice_num  net_value  date             total_month
  _______________________________________________________
  1  100          100.00     21.01.2013       500.00
  2  101          300.00     22.01.2013       500.00
  3  102          100.00     23.01.2013       500.00
  4  103          900.00     01.02.2013       900.00
  .
  .
  .
  </code></pre>
  
  <p>The problem is how to calculate sum of all invoices for a month (i.e. january as above) and display this sum as a value in last column for invoices from january.
  Generaly spoken, I need sum of values of invoices grouped by month but displayed in each row.
  Thanks in advance :) </p>