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

select h1.value - h2.value
from History h1
inner join History h2 on h1.Datetime = '2010-1-1 10:10' and Datetime ='2010-1-1 10:12'