 <p>Total guess:</p> <pre><code>select v1.Value1 - v2.Value2 from (Select Max(Value) as [Value1] from History WHERE Datetime ='2010-1-1 10:10' and tagname ='tag1') as v1 CROSS JOIN ( (Select Max(Value) as [Value2] from History WHERE Datetime ='2010-1-1 10:12' and Tagname ='tag2') as v2) </code></pre>