Question

I am writing a php script to determine the fuel usage of trucks. I use mysql db table for this.

There are several locations that a truck can get fuel, say A, B, C, D locations.
The truck gets fuel from one of these locations which is the closest. And every time the truck gets fuel, the person responsible will enter “the amount of the fuel” and value of “odometer” to program.

sequence_id locations   fuelDispensed   odometer 
1           C           700             8100
2           A           400             9700
3           B           500             15500
4           C           600             17950

and so on.

With this info from db, It is easy to find how many KMs or miles the truck travelled from a location to another just by calculating “odometer” difference between successive rows by using “sequence_id”.

The problem is: People may forget or not be able to enter the values to the program and do it later. the data becomes like this:

sequence_id locations   fuelDispensed   odometer 
1           C           700             8100
2           B           500             15500
3           C           600             17950
4           A           400             9700

In this case, it is not possible to calculate between successive rows based on sequence_id. Maybe, by sorting odometer values ascending and then doing successive calculation between rows seems logical but I could not find out how I can do this.

Edit: My query is something like this:

    SELECT 
    t1.odometer AS km1, 
    t2.odometer AS km2,

    FROM fueldispensed AS t2, fueldispensed AS t1
    WHERE (t1.sequence_id+1= t2.sequence_id) AND (t1.truck_id='$truckid') AND (t2.truck_id='$truckid')  ORDER BY t1.sequence_id";

adding ORDER BY to this query has no effect since I get the succession on “sequence_id”.

Answer 1

Add an ORDER BY to your SQL select statement

ORDER BY odometer ASCENDING

EDIT

OK! I think I understand your problem now.

SELECT t1.truck_id,
       t1.odometer AS km1,  
       MIN(t2.odometer) AS km2
  FROM fueldispensed AS t1, 
       fueldispensed AS t2 
 WHERE t2.truck_id = t1.truck_id
   AND T2.odometer > t1.odometer
 ORDER BY t1.truck_id,
          t1.odometer 
 GROUP BY t1.truck_id,
          t1.odometer 

Should give you something that will work, though not as efficient as it could be

Edit your truck_id selection into the query as appropriate