title: MySQL select and calculate value from multiple columns
body: <p>I am trying to create a mysql select query of which calculates a value based on two other fields.</p>
  
  <p>This is my query,</p>
  
  <pre><code>SELECT request_id, (
  unit_cost * quantity
  ) AS claim_value
  FROM  `xx_non_part_usage` 
  WHERE request_id = request_id
  GROUP BY request_id
  </code></pre>
  
  <p>The query above only brings back the total_value for one of the rows.</p>
  
  <p>For example - here is some sample data,</p>
  
  <pre><code>ID      REQUEST_ID      QUANTITY        UNIT_VALUE
  1       10001           2.0             3.00
  2       10001           1.0             19.00
  3       10003           0.5             18.00
  4       10001           10.0            12.00
  5       10003           0.75            6.76
  6       10002           9.0             3.20
  7       10001           0.10            13.80
  8       10001           1.0             90.99
  9       10004           6.75            3.00
  10      10009           3.23            87.00 
  </code></pre>
  
  <p>As you can see there are several rows of REQUEST_ID '10001'. What the query needs to do is REQUEST_ID * QUANTITY then group them so it only returns the final value price (adding all of the results from the multiply sum (REQUEST_ID * QUANTITY).</p>
  
  <p>Here is an expected result of what I am hoping to get (different example on REQUEST_ID 10003,</p>
  
  <pre><code>REQUEST_ID      TOTAL_VALUE
  10003           14.07
  10004           20.75
  ...
  ...
  </code></pre>
  
  <p>Thank you in advance.</p>