Difference of last 2 rows

Question

Hi i have a table which includes meter reading of different meters. When i give the meter id i need to take the last 2 meter reading values . My table structure and datas are
CREATE TABLE MeterReading
ReadingID int,
MeterID int,
DataCounter int,
Value numeric(16,4)
INSERT INTO MeterReading
VALUES(1, 1, 1, 102.45)
INSERT INTO MeterReading
VALUES(2, 2, 1, 110.45)
INSERT INTO MeterReading
VALUES(3, 1, 2, 110.58)
INSERT INTO MeterReading
VALUES(4, 2, 2, 115.89)
INSERT INTO MeterReading
VALUES(5, 1, 4, 120.00)
INSERT INTO MeterReading
VALUES(6, 2, 4, 116.23)
Here i need the difference of latest (based on DataCounter) difference of Values corresponding MeterID
select top 2 * from MeterReading
where MeterID = 1
order by DataCounter desc
I give the select query. so that i need to take the diffrence of these selected rows' Values. Always the latest value is larger

Answer 1

Sorry friends. I didn't got your way. Here i explain my scenario. Please advice me the correct way so that i can change more cursor queries in my work. I also knew that cursor is not good but not got any suitable approach for this.
Scenario:
   I have 3 table named Table1, Table2 and Table3.
Table 1 is the log table which the data is inserted by windows service from third party.This table logs all the data from the service so that same data can be duplicated. Because third party send the data 3 times to avoid data loss. Windows service send
the acknowledge but some time if the acknowledge delayed or failed seconds time the same data cam,e and log here. The table have 'ProcessStatus' which have value '0'when data is inserted.
To process log table (table1) i need to write one procedure and configure the procedure as SQL job. Here is all the logic.
Step1 : I need to iterate all the rows in table1 which have processstatus is '0'
step2 :Based on the data type (this is column in the table1) insertion is in different table and logic is different.
If data type 1
Step 2.1 First need to check whether data is already exist in the intermediate table (table2). If not found insert the data to table2. Otherwise insert error in the error log table
Step 2.2 Take the last entries from the table2 (based on the constraints like gave in the query question..eg: MeterID) and found the difference of values and insert one record in the table3 which includes the difference, start time as first entry time and
end time as last entry time. So the table 3 have energy consumption.
Step 3....Like data type 2 etc...
Step 4: Update log table record's processstatus as 1.
I think you got the scenario. Please advise me how i change these much operations from the cursor to each set queries