title: Calculating Cumulative Sum in PostgreSQL
body: <p>I want to find the cumulative or running amount of field and insert it from staging to table. My staging structure is something like this:</p>
  
  <pre><code>ea_month    id       amount    ea_year    circle_id
  April       92570    1000      2014        1
  April       92571    3000      2014        2
  April       92572    2000      2014        3
  March       92573    3000      2014        1
  March       92574    2500      2014        2
  March       92575    3750      2014        3
  February    92576    2000      2014        1
  February    92577    2500      2014        2
  February    92578    1450      2014        3          
  </code></pre>
  
  <p>I want my target table to look something like this:</p>
  
  <pre><code>ea_month    id       amount    ea_year    circle_id    cum_amt
  February    92576    1000      2014        1           1000 
  March       92573    3000      2014        1           4000
  April       92570    2000      2014        1           6000
  February    92577    3000      2014        2           3000
  March       92574    2500      2014        2           5500
  April       92571    3750      2014        2           9250
  February    92578    2000      2014        3           2000
  March       92575    2500      2014        3           4500
  April       92572    1450      2014        3           5950
  </code></pre>
  
  <p>I am really very much confused with how to go about achieving this result. I want to achieve this result using PostgreSQL.</p>
  
  <p>Can anyone suggest how to go about achieving this result-set?</p>