title: Update empty column with calculated values from SELECT statement?
body: <p>I have a table, 6 columns, the 6th column is empty but the other 5 have data. I used the 4th and 5th columns to calculate a value and then I wish to insert this value into the 6th column.</p>
  
  <p>This is what I have so far:</p>
  
  <pre><code>UPDATE Table_Name 
  SET Change = tab3.Difference
  FROM
      (SELECT COBDate, FileName, ID, ScenarioID FROM Table_Name WHERE COBDate = '2012-05-18' AND FileName = 'GBP.csv') tab0
  INNER JOIN
      (SELECT  tab1.ID, tab1.ScenarioID,tab1.COBDate, tab1.FileName, Val1 - Val2 AS Difference FROM
          (SELECT COBDate, FileName, ScenarioID, ID, CASE WHEN Value IS NULL THEN 0 ELSE Value END AS Val1 FROM Table_Name WHERE COBDate = '2012-05-18' AND FileName = 'GBP.csv') tab1
      JOIN
          (SELECT COBDate, FileName, ScenarioID, ID, CASE WHEN Value IS NULL THEN 0 ELSE Value END AS Val2  FROM Table_Name WHERE COBDate = '2012-05-17' AND FileName = 'GBP.csv') tab2
      ON tab1.ScenarioID = tab2.ScenarioID AND tab1.ID = tab2.ID) tab3
  ON tab0.COBDate = tab3.COBDate
  AND tab0.FileName = tab3.FileName
  AND tab0.ID = tab3.ID
  AND tab0.ScenarioID = tab3.ScenarioID
  </code></pre>
  
  <p>EDIT: The above code hasn't worked. It has set the same value for every single row regardless of the primary key....</p>