StationId | TPTagId
1         |  7
2         |  8
3         |  9

DateAndTime              | TPTagId | Val
2017-01-01 13:00:00.000  |   7     | 500
2017-01-01 13:15:00.000  |   7     | 700
2017-01-01 13:30:00.000  |   7     | 750
2017-01-01 13:23:00.000  |   8     | 610
2017-01-01 13:24:00.000  |   8     | 630
2017-01-01 13:25:00.000  |   8     | 640
2017-01-01 13:49:00.000  |   9     | 888
2017-01-01 13:49:30.000  |   9     | 890
2017-01-01 13:49:45.000  |   9     | 901

StationId |  Difference
1         |  50
2         |  10
3         |  11

select s.StationID, v.DateAndTime, v.Val
from dbo.tblValues v
inner join dbo.tblStation s on s.TPTagId = v.TPTagId
group by s.StationId, v.DateAndTime, v.Val
order by s.StationId, v.DateAndTime desc