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

What is the type of the value column? If it is already an integer just do:

SELECT 

  (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

else you will have to cast it as an integer or whatever numeric type you want