MySQL table value difference related to current date and previous date - mysql

Question

I have some tables that used to manage vehicle details. Related tables as follows:
store_item
+---------+-----------+--------+
| item_id | item_name | status |
+---------+-----------+--------+
| 1 | Diesel | 1 |
| 2 | Petrol | 1 |
+---------+-----------+--------+
tbl_vehicle
+------------+---------------+
| vehicle_id | registered_no |
+------------+---------------+
| 1 | LE-7476 |
| 2 | 270-0523 |
+------------+---------------+
tbl_direct_fuel
+----------------+----------+---------+------------+------------+
| direct_fuel_id | vehicle | orderNo | issue_date | milo_meter |
+----------------+----------+---------+------------+------------+
| 1 | LE-7476 | 173072 | 2019-11-12 | 30,000 |
| 2 | LE-7476 | 173069 | 2019-11-08 | 29,600 |
| 3 | LE-7476 | 173059 | 2019-11-05 | 29,000 |
| 4 | LE-7476 | 173055 | 2019-10-08 | 25,000 |
| 5 | 270-0523 | 173068 | 2019-11-02 | 10,000 |
| 6 | 270-0523 | 173067 | 2019-10-02 | 8,500 |
+----------------+----------+---------+------------+------------+
tbl_direct_fuel_details
+------------------------+----------------+------+----------+------------+
| direct_fuel_details_id | direct_fuel_id | item | fuel_qty | fuel_price |
+------------------------+----------------+------+----------+------------+
| 100 | 1 | 1 | 20 | 105 |
| 101 | 2 | 1 | 15 | 105 |
| 102 | 3 | 1 | 12 | 105 |
| 103 | 4 | 2 | 50 | 165 |
| 104 | 5 | 1 | 25 | 100 |
| 105 | 6 | 2 | 18 | 165 |
+------------------------+----------------+------+----------+------------+
Desired Output
I want to get no_of_kms, vs issued fuel quantities using issue_date & milo_meter (current milo_meter - previous milo_meter). The output as follows :
+----------+-----------+
| vehicle | no_of_kms |
+----------+-----------+
| LE-7476 | 400 |
| LE-7476 | 600 |
| LE-7476 | 4,000 |
| 270-0523 | 1500 |
+----------+-----------+
I used the following query :
select v1.registered_no as vehicle, si.item_name as fuel, df.milo_meter - df.milo_meter as no_of_kms
from (select dfd.item, sum(dfd.fuel_qty) AS qty
from tbl_direct_fuel df
join tbl_direct_fuel_details dfd on df.direct_fuel_id = dfd.direct_fuel_id
join tbl_vehicle v1 on df.vehicle = v1.vehicle_id
where df.status = 1
group by registered_no) dfd
join store_item si on dfd.item = si.item_id
join (select item, sum(dfd.fuel_qty) AS fuel_qty
from tbl_direct_fuel_details
group by item) dfd on si.item_id=dfd.item
But the above query didn't working fine. What may be going wrong ? Can anyone help me ?

Answer 1

If you are running MySQL 8.0, you can do this simply with window function lag():
select *
from (
select
issue_date,
vehicle,
milo_meter
- lag(milo_meter) over(partition by vehicle order by issue_date) no_of_kms
from tbl_direct_fuel
) t
where no_of_kms is not null
order by vehicle desc, issue_date desc
I added the issue_date to the output columns, since this seems like a sensible information to understand the results.
In earlier versions, I think that an inline query might do the trick:
select *
from (
select
issue_date,
vehicle,
milo_meter - (
select max(milo_meter)
from tbl_direct_fuel d1
where d1.vehicle = d.vehicle and d1.issue_date < d.issue_date
) no_of_kms
from tbl_direct_fuel d
) t
where no_of_kms is not null
order by vehicle desc, issue_date desc
This assumes that the milo_meter of a given vehicule can only increase, which seems like a reasonable assumption.
Demo on DB Fiddle
Both queries return:
issue_date | vehicle | no_of_kms
:--------- | :------- | --------:
2019-11-12 | LE-7476 | 400
2019-11-08 | LE-7476 | 600
2019-11-05 | LE-7476 | 4000
2019-11-02 | 270-0523 | 1500