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

Do you really need wrapping select statement?

You can declare two variables @value1 and @value2 and substruct them.

declare @value1 int, @value2 int

select @value1 = Max(Value) as [Value1] from History WHERE Datetime ='2010-1-1 10:10'       and tagname ='tag1'

select @value2 = Max(Value) as [Value2] from History WHERE Datetime ='2010-1-1 10:12'      and Tagname ='tag2'

select @value1 - @value2