Calculate sum when value changes - mysql

Question

I am trying to build a system that will track vehicle fuelings, and have run into a problem with one report; determining fuel efficiency in distance/fuel. Sample data is:
odometer
fuel
partial_fillup
61290
10.3370
0
61542
6.4300
0
61735
4.3600
0
61994
7.5000
0
62242
5.4070
0
62452
8.1100
0
62713
5.7410
1
62876
9.4850
0
63243
6.1370
1
63499
10.7660
0
Where odometer is the total distance the vehicle has traveled, fuel is the number of gallons or liters put in, and partial_fillup is a boolean meaning the fuel tank was not completely filled if non-zero.
If the user fills the tank each time the query I can use is:
set #a = null;
select
odometer,
odometer-previousOdometer distance,
fuel,
(odometer-previousOdometer)/fuel mpg,
partial_fillup
from
(
select
#a as previousOdometer,
#a:=odometer,
odometer,
fuel/1000 fuel,
partial_fillup
from fuel
where
vehicle_id =1
and odometer >= 61290
order by odometer
) as readings
where readings.previousOdometer is not null;
However, when the user only partially fills the tank, the correct procedure would be to subtract the last full fueling from current odometer reading, then divide by the sum of all fuel since the previous odometer reading, so at odometer 63499, the calculate would be (63499-62876)/(10.7660+6.1370)

Answer 1

This will get the average used on the last ride:
select
odometer,
odometer-lag(odometer) over (order by odometer) as distance,
fuel,
(odometer-lag(odometer) over (order by odometer))/fuel as mpg
from fuel
output:
odometer
distance
fuel
mpg
61290
10.3370
61542
252
6.4300
39.1913
61735
193
4.3600
44.2661
61994
259
7.5000
34.5333
62242
248
5.4070
45.8665
62452
210
8.1100
25.8940
62713
261
5.7410
45.4625
62876
163
9.4850
17.1850
63243
367
6.1370
59.8012
63499
256
10.7660
23.7786
Or you can calculate the total drive distance, and the total amount of fuel used:
select
distance,
sum_fuel,
distance/sum_fuel as mpg
from (
select
f.odometer,
f.odometer-(select min(odometer) from fuel) as distance,
fuel,
sum_fuel
from fuel f
inner join (
select
odometer,
sum(fuel) over (order by R) as sum_fuel
from (
select
odometer,
fuel,
row_number() over (order by odometer) R
from fuel) x
) x on x.odometer = f.odometer
) x2
which will get next output, which will get closer to an average after a longer time of measurement:
distance
sum_fuel
mpg
0
10.3370
0.0000
252
16.7670
15.0295
445
21.1270
21.0631
704
28.6270
24.5922
952
34.0340
27.9720
1162
42.1440
27.5721
1423
47.8850
29.7170
1586
57.3700
27.6451
1953
63.5070
30.7525
2209
74.2730
29.7416
DBFIDDLE

Answer 2

I was able to figure it out after studying Luuk's answer. I'm sure there is a more efficient way to do this; I am not used to using variables in SQL. But, the answers are correct in the test data.
set #oldOdometer = null;
set #totalFuel = 0;
select
s.odometer,
format(fuel, 3) fuel,
s.distance,
format( distance / fuel, 2) as mpg
from (
select
partial_fillup as partial,
odometer,
(fuel+#totalFuel) as fuel,
#totalFuel as totalFuel,
#oldOdometer oldOdometer,
if ( partial_fillup, null,odometer - #oldOdometer ) as distance,
#totalFuel := if ( partial_fillup, #totalFuel + fuel, 0) as pastFuel,
#oldOdometer := if (partial_fillup,#oldOdometer,odometer ) as runningOdometer
from
fuel
order by
odometer ) s
where s.distance is not null
order by s.odometer
limit 1,999;
limit 1,999 simply there to skip the first row returned, since there is not enough data to calculate distance or mpg. On my copy of MySQL, doing this means you do not need to initialize the two variables (you don't have to include the set commands at the beginning), so it works with my reporting tool very well. If you do initialize them, you do not need the limit statement. Works assuming you don't have more than 999 rows returned.