mysql many-to-one and group by giving unusual results - mysql

Question

im having difficulty with the following fairly simple setup:
CREATE TABLE IF NOT EXISTS invoices (
id int(11) NOT NULL auto_increment,
PRIMARY KEY (id)
);
CREATE TABLE IF NOT EXISTS invoices_items (
id int(11) NOT NULL auto_increment,
invoice_id int(11) NOT NULL,
description text NOT NULL,
amount decimal(10,2) NOT NULL default '0.00',
PRIMARY KEY (id)
);
CREATE TABLE IF NOT EXISTS invoices_payments (
id int(11) NOT NULL auto_increment,
invoice_id int(11) NOT NULL,
amount decimal(10,2) NOT NULL default '0.00',
PRIMARY KEY (id)
);
some data:
INSERT INTO invoices (id) VALUES (1);
INSERT INTO invoices_items (id, invoice_id, description, amount) VALUES
(1, 1, 'Item 1', '750.00'),
(2, 1, 'Item 2', '750.00'),
(3, 1, 'Item 3', '50.00'),
(4, 1, 'Item 4', '150.00');
INSERT INTO invoices_payments (id, invoice_id, amount) VALUES
(1, 1, '50.00'),
(2, 1, '1650.00');
and the sql yielding unusual results:
select invoices.id,
ifnull(sum(invoices_payments.amount),0) as payments_total,
ifnull(count(invoices_items.id),0) as item_count
from invoices
left join invoices_items on invoices_items.invoice_id=invoices.id
left join invoices_payments on invoices_payments.invoice_id=invoices.id
group by invoices.id
results in the (erroneous) output
id payments_total item_count
1 6800.00 8
now, as evidenced by there being infact only four 'invoice_item' rows, i dont understand why mysql is not grouping properly.
EDIT
i know i can do something like this:
select x.*, ifnull(sum(invoices_payments.amount),0) as payments_total from (
select invoices.id,
ifnull(count(invoices_items.id),0) as item_count
from invoices
left join invoices_items on invoices_items.invoice_id=invoices.id
group by invoices.id
) as x left join invoices_payments on invoices_payments.invoice_id=x.id
group by x.id
but i want to know if im doing something wrong in the first query - i cant immediately see why the first query is giving incorrect results! :(

Answer 1

Your join logic is incorrect. In your join, you specify invoices_items.invoice_id = invoices.id. You also specify invoices_payments.invoice_id = invoices.id. Because of transitivity, you end up with:
invoices_items.invoice_id = invoices.id
invoices_payments.invoice_id = invoices.id
invoice_items.invoice_id = invoices_payments.invoice_id
The sum of the 2 invoice payments is $1700. For every invoice payment, there are 4 invoice_items that satisfy the above relations. $1700 * 4 = $6800.
For every invoice item, there will be two invoice payments that satisfy the above relations. 4 invoice items * 2 = 8 count.

Answer 2

There are two tables with a many:one relationship with invoices. Your count is the cartesian product.
The payments should be applied to the invoice, not the invoice items. Get the invoice total first, then join the payments to it.
This may be similar to what you are looking for:
SELECT
invoice_total.invoice_id,
invoice_total.amount as invoice_amount,
payments_total.amount as total_paid
FROM
(
SELECT
invoice_id,
SUM(amount) as amount
FROM
invoices_items
GROUP BY
invoice_id
) invoice_total
INNER JOIN
(
SELECT
invoice_id,
SUM(amount) as amount
FROM
invoices_payments
GROUP BY
invoice_id
) payments_total
ON invoice_total.invoice_id = payments_total.invoice_id;

Answer 3

edit:
ah, sorry - see your point now. The reason you're getting unexpected results is that this query:
SELECT *
FROM invoices
LEFT JOIN invoices_items ON invoices_items.invoice_id = invoices.id
LEFT JOIN invoices_payments ON invoices_payments.invoice_id = invoices.id;
results in this:
id id invoice_id description amount id invoice_id amount
1 1 1 Item 1 750.00 1 1 50.00
1 1 1 Item 1 750.00 2 1 1650.00
1 2 1 Item 2 750.00 1 1 50.00
1 2 1 Item 2 750.00 2 1 1650.00
1 3 1 Item 3 50.00 1 1 50.00
1 3 1 Item 3 50.00 2 1 1650.00
1 4 1 Item 4 150.00 1 1 50.00
1 4 1 Item 4 150.00 2 1 1650.00
As you can see you get every invoices_items record once each for every invoices_payments record. You're going to have to grab (i.e. group) them separately.
Note that the GROUP BY clause in your initial query is redundant.
Here's what you need:
SELECT
invoices.id,
payments_total.payments_total,
IFNULL(COUNT(invoices_items.id),0) AS item_count
FROM invoices
LEFT JOIN invoices_items ON invoices.id = invoices_items.invoice_id
LEFT JOIN (
SELECT invoice_id,
IFNULL(SUM(invoices_payments.amount),0) AS payments_total
FROM invoices_payments
GROUP BY invoice_id
) AS payments_total ON invoices.id = payments_total.invoice_id
;