title: how do I subtract values from two select statements
body: <p>I would like to subtract one value from another value. The schema of the  table is as follows:</p>
  
  <pre><code>   tag, datetime,value
     ------------
     tag1, 2010-1-1 10:10:00, 123
     tag2, 2010-2-2 10:12:00. 321
  
    select * from  
  
    ( (Select    Max(Value) as [Value1] from History WHERE Datetime ='2010-1-1 10:10'       and tagname ='tag1') as v1   -
  
    (  (Select    Max(Value) as [Value2] from History WHERE Datetime ='2010-1-1 10:12'      and Tagname ='tag2')  as v2))
  </code></pre>
  
  <p>obviously I am lost...how do I do this.</p>
  
  <p>thanks</p>
  
  <p>ms-sql</p>