title: DateDiff of 2 dates in the same column SQL
body: <p>Using SQL Server 2005 I need to get the <code>datediff</code> of 2 dates that are in the same column, table looks like this:</p>
  
  <pre><code>OrderNo    OpNo    ResType    LoadedStartDate
  -----------------------------------------------------
  12345      1       PAINT      2014-05-01 00:00:00.000
  12345      2       PAINT      2014-05-02 00:00:00.000
  12345      3       PAINT      2014-05-03 00:00:00.000
  12345      4       ASMB       2014-05-04 00:00:00.000
  67890      1       PAINT      2014-05-02 00:00:00.000
  67890      2       PAINT      2014-05-03 00:00:00.000
  67890      3       PAINT      2014-05-04 00:00:00.000
  67890      4       ASMB       2014-05-05 00:00:00.000
  </code></pre>
  
  <p>I need to get the date diff of OpNo 1 and OpNo 4 where they are the same order number. The OpNo will always be 1 and 4 as the ones I am trying to compare, as will the ResType.</p>
  
  <p>Output would need to look like this:</p>
  
  <pre><code>OrderNo    Difference
  ----------------------
  12345      3
  67890      3
  </code></pre>
  
  <p>Thanks for any help :)</p>