MySQL: How to sum distinct rows in complex joined query - mysql

Question

I have MySQL question I cannot solve myself (for the first time).
I have a query-with-parameters database plus PHP program that, together, generate extensive MySQL queries to run.
The problem is actually a simple one: that of correct summation. I need to SUM distinct rows (not values) within a complex, multi-joined query, and I cannot get it to work.
Do not ask why I work with the data structure below - I am working with data that is supplied to me and it needs to be this way. (The tables represent existing invoices.)
I will try to reproduce the situation very simplified here.
TABLE INVOICE
=============
Inv.Nr (ID) Other Data
------------------------
#1 Stuff
#2 Stuff
#3 More Stuff
TABLE INVOICE LINE
==================
ID Inv.Nr QUANTITY ArticleID UNIT PRICE
----------------------------------------------
1 #1 1 5 € 2.50
2 #1 1 109 € 4.00
3 #2 4 77 € 5.00
4 #2 10 91 € 6.00
TABLE INVOICE LINE VAT
======================
ID LINE-ID AMOUNT VATP VAT
1 1 € 2.00 25% € 0.50
2 2 € 2.00 25% € 0.50
3 2 € 1.42 6% € 0.08
4 3 €18.87 6% € 1.23
5 4 €16.00 25% € 4.00
6 4 €37.74 6% € 2.26
As you can see: some articles have a double VAT rate, because they consist of more elements that have different VAT rates (i.e. a book with a cd).
Now the queries are very long, there are much more tables joined that can have dynamic WHERE and GROUP BY clauses. So a query might look somewhat like (again much simplified):
SELECT `Inv.Nr`, ArticleID, SUM(Quantity), SUM(Amount), SUM(VAT)
FROM ((((`Invoice` INNER JOIN `Invoice Line`
ON `Invoice`.`Inv.Nr`=`Invoice Line`.`Inv.Nr`)
INNER JOIN `Invoice Line VAT`
ON `Invoice Line`.ID = `Invoice Line VAT`.`Line-ID`)
INNER JOIN `More Stuff`
ON .... )
INNER JOIN ....
ON ..... )
WHERE ....
GROUP BY .....
HAVING .....
The INNER JOINs defined by ... are many to 1, so Invoice Line VAT is on the many-side of both its JOIN relations.
The WHERE, GROUP BY and HAVING are semi-dynamically created in PHP code.
My problem is that i cannot get a proper SUM(Amount) and SUM(Quantity) at the same time, since the Quantity is added multiple times if there are multiple VAT rates to one invoice line.
SUM(DISTINCT Quantity) obviously doesn't work, since I need distinct rows, not values.
I cannot really create a subquery that either calculates the number of VAT rates (and divides the SUM(Quantity)), or calculates the Amount, since the subquery needs the same WHERE/HAVING parameters as the main query to work properly, and those are semi-dynamic (the queries are in a database and contain parameters that are filled in following the user's commands). Well, to be fair, I could do it, but it would leave the query-database and the php software extremely complicated, and I don't want to use a very complex solution for such a very simple problem, especially since someone else will have to maintain it in the future.
So how do I:
SUM the quantity only on distinct rows, or
COUNT the number of VAT rates per line, given the WHERE/HAVING (so without a subquery)?
I could add extra fields to the tables to help with this problem, but that possibility didn't help me - yet. For instance: storing the number of VAT rates doesn't help, since in the WHERE there may be a selection on VAT rate.
I hope it is something VERY simple that I overlooked, but I have been searching for hours now to no avail...
If anyone can help me that would be great! Thanks in advance!
EDIT: I found a solution, but I am not very pleased with it. I have to split up the WHERE, and SUM SUMs and repeat columns... It is UGLY and badly maintainable.
It is as follows:
SELECT `Inv.Nr`, ArticleID, SUM(Quantity), SUM(Amount), SUM(VAT)
FROM ((`Invoice` INNER JOIN `Invoice Line`
ON `Invoice`.`Inv.Nr`=`Invoice Line`.`Inv.Nr`)
INNER JOIN
(SELECT SUM(Amount) AS Amount, SUM(VAT) AS VAT, `Line-ID`
FROM ((`Invoice Line VAT`
INNER JOIN `More Stuff`
ON .... )
INNER JOIN ....
ON ..... )
WHERE some-where-stuff
GROUP BY `Line-ID`) x
ON `Invoice Line`.ID = x.`Line-ID`)
WHERE other-where-stuff
GROUP BY .....
HAVING .....
I hope someone got a more elegant, simpler solution!

Answer 1

In an update to the question, I answered the question myself. I said that I hoped for a less ugly and badly maintainable solution than:
SELECT `Inv.Nr`, ArticleID, SUM(Quantity), SUM(Amount), SUM(VAT)
FROM ((`Invoice` INNER JOIN `Invoice Line`
ON `Invoice`.`Inv.Nr`=`Invoice Line`.`Inv.Nr`)
INNER JOIN
(SELECT SUM(Amount) AS Amount, SUM(VAT) AS VAT, `Line-ID`
FROM ((`Invoice Line VAT`
INNER JOIN `More Stuff`
ON .... )
INNER JOIN ....
ON ..... )
WHERE some-where-stuff
GROUP BY `Line-ID`) x
ON `Invoice Line`.ID = x.`Line-ID`)
WHERE other-where-stuff
GROUP BY .....
HAVING .....
It turns out, that, now that I am working with this solution and rephrasing all my queries based in it, it is not so humongous and ugly after all. It turns out that it works quite well and much better than other solutions and workarounds I have tried. Because I guess there is no other solution than what I wrote I close this question by answering that above cited answer is the right one.
It turns out that using the correct SQL code instead of workarounds is the right way to do, even when it looks too complicated at first. And since there is nothing like SUM(DISTINCT ...) that works with distinct records instead of values, in this case the above code is the correct code.