MySQL- Getting a SUM when a variable inside it doesn't exist - mysql

Question

first post here!
So as a young Padawan in SQL and Databases in general I am working on creating a database for a business to manage orders/items/prices etc. I want to create a View that from 3 linked tables {items,prices,discounts}(see tables at the bottom) calculates and show the total price of an item. As a rule, the discount column shouldn't be zero( because we only want in the table discounts entries with actual discount , not 0 )
From the following entries I want to show all of them but my view only shows the one's with discounts.
insert into items (`item_id`,`item_name`, `item_quantity`) values
(102,'item1',20),
(103,'item2',20),
(404,'item3',20); # <-- It won't be shown if I do SELECT * FROM view;
insert into discounts (`item_id`,`discount`) values
(102,50),
(103,25);
insert into prices (`item_id`,`price`) values
(102,100),
(103,100),
(404,100);
And here is my View:
CREATE VIEW ItemsPrice AS
SELECT
i.item_id,
i.item_name,
SUM((1-d.discount/100)*p.price*i.item_quantity)
FROM
items AS i
INNER JOIN
prices AS p ON i.item_id=p.item_id
INNER JOIN
discounts AS d ON (p.item_id=d.item_id)
GROUP BY item_id
ORDER BY total;
Here are my tables (just in case I made them wrong):
DROP TABLE IF EXISTS `items`;
CREATE TABLE `items` (
`item_id` int(30) NOT NULL,
`item_name` varchar(35) NOT NULL,
`item_quantity` double(25,0) ,
PRIMARY KEY (`item_id`)
);
#2=======
DROP TABLE IF EXISTS `prices`;
CREATE TABLE `prices` (
`item_id`int(30) NOT NULL,
`price` decimal(30,2) NOT NULL,
PRIMARY KEY (`item_id`),
CONSTRAINT `prices_ibfk_1` FOREIGN KEY (`item_id`) REFERENCES `items` (`item_id`)
);
#3=======
DROP TABLE IF EXISTS `discounts`;
CREATE TABLE `discounts` (
`item_id` int(30) NOT NULL,
`discount` int(3) NOT NULL,
PRIMARY KEY (`item_id`),
CONSTRAINT `discount_ibfk_1` FOREIGN KEY (`item_id`) REFERENCES `items` (`item_id`)
);
Sorry for not providing a schema. Don't know how to make one.
Hope I didn't waste much of your time! You are my hero.

Answer 1

You just need a left join - this will grab all items with prices even of they do not have a discount. You use COALESCE to replace the NULL discounts with a valid number.
SELECT
i.item_id,
i.item_name,
COALESCE(d.discount,0) as discount,
p.price,
i.item_quantity
FROM
items AS i
INNER JOIN
prices AS p ON i.item_id=p.item_id
LEFT JOIN
discounts AS d ON (p.item_id=d.item_id)
GROUP BY item_id
ORDER BY total;
Your query with COALESCE:
SELECT
i.item_id,
i.item_name,
(1-COALESCE(d.discount,0)/100)*p.price*i.item_quantity) as totalAmount
FROM
items AS i
INNER JOIN
prices AS p ON i.item_id=p.item_id
LEFT JOIN
discounts AS d ON (p.item_id=d.item_id)
GROUP BY item_id
ORDER BY total;