SQL 'merging' columns from result sets into one result set - mysql

Question

I've been pulling my hair about how to write a particular view within the constraints of MySQL.
The following tables and columns are of importance:
CREATE TABLE `invoices` (
`id` int(10) unsigned NOT NULL AUTO INCREMENT,
PRIMARY KEY (`id`)
)
-- Joins payments to invoices. The sum of all `invoice_currency_value`s is the balance paid towards an invoice.
CREATE TABLE `financial_transactions_invoices` (
`id` int(10) unsigned NOT NULL AUTO INCREMENT,
`invoice` int(10) unsigned NOT NULL,
`invoice_currency_value` decimal(8,2) unsigned NOT NULL,
PRIMARY KEY (`id`)
)
-- Lists items (services) available to purchase.
CREATE TABLE `items` (
`id` int(10) unsigned NOT NULL AUTO INCREMENT,
`value` decimal(8,2) unsigned NOT NULL
PRIMARY KEY (`id`)
)
-- Each instance represents that the `item` has been purchased.
CREATE TABLE `item_instances` (
`id` int(10) unsigned NOT NULL AUTO INCREMENT,
`invoice` int(10) unsigned NOT NULL,
`item` int(10) unsigned NOT NULL,
`invoice_currency_rate` decimal(11,5) unsigned NOT NULL,
PRIMARY KEY (`id`)
)
-- Any number of tax instances can exist for an item instance and indicate this tax has been applied to the associated item instance.
CREATE TABLE `tax_instances` (
`id` int(10) unsigned NOT NULL AUTO INCREMENT,
`item_instance` int(10) unsigned NOT NULL,
`value` decimal(8,2) unsigned NOT NULL,
PRIMARY KEY (`id`)
)
Now, I need a view that lists for each row,
the invoice number
the total value of the invoice
the total tax on the invoice
and the total value of payments made towards the invoice
However, I can't figure out how to get these three separate queries into the same result set of one row per invoice, e.g.
inv_no total_value total_tax payments
1 150 5 120
2 120 10 20
3 10 0 10
4 1000 150 1150
I have written the following query which produces the desired result, but due to the 'no subquery' rule in MySQL views, it is not acceptable.
SELECT `invoice_id`, SUM(`total_value`) AS `total_value`, SUM(`total_tax`) AS `total_tax`,
SUM(`paid_balance`) AS `paid_balance`
FROM
(SELECT `invoices`.`id` AS `invoice_id`, SUM(`items`.`value` * `item_instances`.`invoice_currency_rate`) AS `total_value`,
NULL AS `total_tax`, NULL AS `paid_balance`
FROM `items`
JOIN `item_instances` ON `items`.`id` = `item_instances`.`item`
JOIN `invoices` ON `item_instances`.`invoice` = `invoices`.`id`
GROUP BY `invoices`.`id`
UNION
SELECT `invoices`.`id`, NULL, SUM(`tax_instances`.`value`), NULL
FROM `tax_instances`
JOIN `item_instances` ON `tax_instances`.`item_instance` = `item_instances`.`id`
JOIN `invoices` ON `item_instances`.`invoice` = `invoices`.`id`
GROUP BY `invoices`.`id`
UNION
SELECT `invoices`.`id`, NULL, NULL, SUM(`financial_transactions_invoices`.`invoice_currency_value`)
FROM `financial_transactions_invoices`
JOIN `invoices` ON `financial_transactions_invoices`.`invoice` = `invoices`.`id`
GROUP BY `invoices`.`id`) AS `components`
GROUP by `invoice_id`;
Without tackling the problem in the way I have, I can't think of any other way I can do it within MySQL.
Any ideas? Appreciate any help.

Answer 1

You could create two views. One with the UNION Subquery, and one with the outer query.