Incorrect values with multiple left joins (MySQL) - mysql

Question

I am trying to make a report. It is supposed to give me a list of the machines at a specific customer and the sum of hours and material that was put in to that machine.
In the following examples, I select the sum of materials and hours in different fields to make the problem clearer. But i really want to sum the material an hours, then group them by the machine field.
I can query the list of machine and cost of hours without problems.
SELECT CONCAT(`customer`.`PREFIX`, `wo`.`machine_id`) AS `machine`,
ROUND(COALESCE(SUM(`wohours`.`length` * `wohours`.`price`), 0), 2) AS `hours`
FROM `wo`
JOIN `customer` ON `customer`.`id`=`wo`.`customer_id`
LEFT JOIN `wohours` ON `wohours`.`wo_id`=`wo`.`id` AND `wohours`.`wo_customer_id`=`wo`.`customer_id`
AND `wohours`.`wo_machine_id`=`wo`.`machine_id` AND `wohours`.`date`>=(CURDATE() - INTERVAL DAY(CURDATE() - INTERVAL 1 DAY) DAY) - INTERVAL 11 MONTH
WHERE `wo`.`customer_id`=1
GROUP BY `wo`.`machine_id`;
This gives me the correct values for hours. But when I add the material like this:
SELECT CONCAT(`customer`.`PREFIX`, `wo`.`machine_id`) AS `machine`,
ROUND(COALESCE(SUM(`wohours`.`length` * `wohours`.`price`), 0), 2) AS `hours`,
ROUND(COALESCE(SUM(`womaterial`.`multiplier` * `womaterial`.`price`), 0), 2) AS `material`
FROM `wo`
JOIN `customer` ON `customer`.`id`=`wo`.`customer_id`
LEFT JOIN `wohours` ON `wohours`.`wo_id`=`wo`.`id` AND `wohours`.`wo_customer_id`=`wo`.`customer_id`
AND `wohours`.`wo_machine_id`=`wo`.`machine_id` AND `wohours`.`date`>=(CURDATE() - INTERVAL DAY(CURDATE() - INTERVAL 1 DAY) DAY) - INTERVAL 11 MONTH
LEFT JOIN `womaterial` ON `womaterial`.`wo_id`=`wo`.`id` AND `womaterial`.`wo_customer_id`=`wo`.`customer_id`
AND `womaterial`.`wo_machine_id`=`wo`.`machine_id` AND `wohours`.`date`>=(CURDATE() - INTERVAL DAY(CURDATE() - INTERVAL 1 DAY) DAY) - INTERVAL 11 MONTH
WHERE `wo`.`customer_id`=1
GROUP BY `wo`.`machine_id`;
then both hour and material values are incorrect.
I have read other threads where people with similar problems could solve this by splitting it in multiple queries or subqueries. But I don't think that is possible in this case.
Any help is appreciated.
//John

Answer 1

Your other reading is correct. You will need to put them into their own "subquery" for the join. The reason you are probably getting invalid values is that the materials table has multiple records per machine, thus causing a Cartesian result from your original based on hours. And you don't know which has many vs just one making it look incorrect.
So, I've written, and each inner-most query for pre-aggregating the woHours and woMaterial will produce a single record per "wo_id and machine_id" to join back to the wo table when finished. Each of these queries has the criteria on the single customer ID you are trying to run it for.
Then, as re-joined to the work order (wo) table, it grabs all records and applies the ROUND() and COALESCE() in case no such hours or materials present. So this is a return of something like
WO Machine ID Machine Hours Material
1 1 CustX 1 2 0
2 4 CustY 4 2.5 6.5
3 4 CustY 4 1.2 .5
4 1 CustX 1 1.5 1.2
Finally, you can now roll up the SUM() of all these entries into a single row per machine ID
Machine Hours Material
CustX 1 3.5 1.2
CustY 4 3.7 7.0
SELECT
AllWO.Machine,
SUM( AllWO.Hours ) Hours,
SUM( AllWO.Material ) Material
from
( SELECT
wo.wo_id,
wo.Machine_ID,
CONCAT(customer.PREFIX, wo.machine_id) AS machine,
ROUND( COALESCE( PreSumHours.MachineHours, 0), 2) AS hours,
ROUND( COALESCE( PreSumMaterial.materialHours, 0), 2) AS material
FROM
wo
JOIN customer
ON wo.customer_id = customer.id
LEFT JOIN ( select wohours.wo_id,
wohours.wo_machine_id,
SUM( wohours.length * wohours.price ) as machinehours
from
wohours
where
wohours.wo_customer_id = 1
AND wohours.date >= ( CURDATE() - INTERVAL DAY( CURDATE() - INTERVAL 1 DAY) DAY) - INTERVAL 11 MONTH
group by
wohours.wo_id,
wohours.wo_machine_id ) as PreSumHours
ON wo.id = PreSumHours.wo_id
AND wo.machine_id = PreSumHours.wo_machine_id
LEFT JOIN ( select womaterial.wo_id,
womaterial.wo_machine_id,
SUM( womaterial.length * womaterial.price ) as materialHours
from
womaterial
where
womaterial.wo_customer_id = 1
AND womaterial.date >= ( CURDATE() - INTERVAL DAY( CURDATE() - INTERVAL 1 DAY) DAY) - INTERVAL 11 MONTH
group by
womaterial.wo_id,
womaterial.wo_machine_id ) as PreSumMaterial
ON wo.id = PreSumMaterial.wo_id
AND wo.machine_id = PreSumMaterial.wo_machine_id
WHERE
wo.customer_id = 1 ) AllWO
group by
AllWO.Machine_ID