How do i write a group by query in PostgreSQL - postgresql

Question

I'm getting errors with PostgreSQL when am writing a group by query,
am sure someone will tell me to put all the columns I've selected in group by, but that will not give me the correct results.
Am writing a query that will select all the vehicles in the database and group the results by vehicles, giving me the total distance and cost for a given period.
Here is how am doing the query.
SELECT i.vehicle AS vehicle,
i.costcenter AS costCenter,
i.department AS department,
SUM(i.quantity) AS liters,
SUM(i.totalcost) AS Totalcost,
v.model AS model,
v.vtype AS vtype
FROM fuelissuances AS i
LEFT JOIN vehicles AS v ON i.vehicle = v.id
WHERE i.dates::text LIKE '%2019-03%' AND i.deleted_at IS NULL
GROUP BY i.vehicle;
If I put all the columns that are in the select in the group bt, the results will not be correct.
How do i go about this without putting all the columns in group by and creating sub-queries?
The fuel table looks like:
vehicle dates department quantity totalcost
1 2019-01-01 102 12 1200
1 2019-01-05 102 15 1500
1 2019-01-13 102 18 1800
1 2019-01-22 102 10 1000
2 2019-01-01 102 12 1260
2 2019-01-05 102 19 1995
2 2019-01-13 102 28 2940
Vehicle Table
id model vtype
1 1 2
2 4 6
2 5 7
This is the results i expect from the query
vehicle dates department quantity totalcost model vtype
1 2019-01-01 102 12 1200 1 2
1 2019-01-05 102 15 1500 1 2
1 2019-01-13 102 18 1800 1 2
1 2019-01-22 102 10 1000 1 2
1 2019-01-18 102 10 1000 1 2
1 65 6500
2 2019-01-01 102 12 1260 5 7
2 2019-01-05 102 19 1995 5 7
2 2019-01-13 102 28 2940 5 7
1 45 6195

Answer 1

Your query doesn't really make sense. Apparently there can be multiple departments and costcenters per vehicle in the fuelissuances table - which of those should be returned?
One way to deal with that, is to return all of them, e.g. as an array:
SELECT i.vehicle,
array_agg(i.costcenter) as costcenters,
array_agg(i.department) as departments,
SUM(i.quantity) AS liters,
SUM(i.totalcost) AS Totalcost,
v.model,
v.vtype
FROM fuelissuances AS i
LEFT JOIN vehicles AS v ON i.vehicle = v.id
WHERE i.dates >= date '2019-03-01'
and i.date < date '2019-04-01'
AND i.deleted_at IS NULL
group by i.vehicle, v.model, v.vtype;
Instead of an array, you could also return a comma separated lists of those values, e.g. string_agg(i.costcenter, ',') as costcenters.
Adding the columns v.model and v.vtype won't (shouldn't) change anything as the group by i.vehicle will only return a single vehicle anyway and thus the model and vtype won't change for that in the group.
Note that I removed the useless aliases and replaced the condition on the date with a proper range condition that can make use of an index on the dates column.
Edit
Based on your new sample data, you want a running total, rather than a "regular" aggregation. This can easily be done using window functions
SELECT i.vehicle,
i.costcenter,
i.department,
SUM(i.quantity) over (w) AS liters,
SUM(i.totalcost) over (w) AS Totalcost,
v.model,
v.vtype
FROM fuelissuances AS i
LEFT JOIN vehicles AS v ON i.vehicle = v.id
WHERE i.dates >= date '2019-01-01'
and i.dates < date '2019-02-01'
AND i.deleted_at IS NULL
window w as (partition by i.vehicle order by i.dates)
order by i.vehicle, i.dates;
I would not create those "total" lines using SQL, but rather in your front end that display the data.
Online example: https://rextester.com/CRJZ27446

Answer 2

You need to use a nested query to get those SUM you want inside that query.
SELECT i.vehicle AS vehicle,
i.costcenter AS costCenter,
i.department AS department,
(SELECT SUM(i.quantity) FROM TABLES WHERE CONDITIONS GROUP BY vehicle) AS liters,
(SELECT SUM(i.totalcost) FROM TABLES WHERE CONDITIONS GROUP BY vehicle) AS Totalcost,
v.model AS model,
v.vtype AS vtype
FROM fuelissuances AS i
LEFT JOIN vehicles AS v ON i.vehicle = v.id
WHERE i.dates::text LIKE '%2019-03%' AND i.deleted_at IS NULL;