how do I subtract values from two select statements

Question

I would like to subtract one value from another value. The schema of the table is as follows:

   tag, datetime,value
   ------------
   tag1, 2010-1-1 10:10         


        

Answer 1

Total guess:

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)