Question

I am having difficulties subtracting data from the same column. My data structure looks like this:

TimeStamp  ID  state  Gallons
1/01/2012   1    NY     100
1/05/2012   1    NY      90
1/10/2012   1    NY      70
1/15/2012   1    NY      40

So I would like to get the result of (100-40) = 60 gallons used for that time period. I tried using the following query:

SELECT T1.Gallons, T1.Gallons -T1.Gallons AS 'Gallons used'
FROM Table 1 AS T1 
where Date = '2012-07-01'
ORDER BY Gallons

It seems like a simple way of doing this. However, I can not find an efficient way of including the timestamp as a way of retrieving the right values (ex. gallon value on 1/1/2012 – gallon value on 1/30/2012).

ps. The value of “Gallons” will always decrease

Answer 1

select max(gallons) - min(gallons)
from table1 
group by Date
having Date = '2012-07-01'