Is there a way to calculate total from a MySQL Query? - mysql

Question

I'm trying to build a cart system. So far, I can pull the user cart data easily with some join and even calculate the total price based on unit price and quantity user has in another column. However, I wanted to know if there is a way to actually compute the total amount owed directly from an SQL query. Allow me to showcase an example. Here are my tables:
products
product_id | product_name | product_price | stock
1 | Pasta | 10 | 50
2 | Bread | 2 | 100
cart
user_id | product_id | quantity
1 | 1 | 3
1 | 2 | 2
My Current Query
SELECT
`cart`.`product_id`,
`cart`.`user_id`,
`cart`.`quantity`,
`products`.`product_price` as `unit_price`,
`products`.`product_name`,
`cart`.`quantity` * `products`.`product_price` as `Total`
FROM `cart`
INNER JOIN `products` ON `products`.`product_id` = `cart`.`product_id`
WHERE `cart`.`u_id` = 1;
As you can see, the query above will work and return me all the products in the cart of a specific user and add the Total column to the result with the total price for each item.
Now, if I want to calculate the gross total, I have to read each row in my PHP code and do the total. Although I can do this, I was wondering if MySQL has a way of returning just the gross total directly through a single query.
Bonus question: Is a cart table structure like the one above good enough?

Answer 1

You can use "SUM" Operator in MYSQL to get the value from database directly.
Here is a documentation where you can get more info:
https://www.mysqltutorial.org/mysql-sum/
Regarding the code here is an example:
SELECT SUM(`cart`.`quantity` * `products`.`product_price`) as `CartTotal`
FROM `cart`
INNER JOIN `products` ON `products`.`product_id` = `cart`.`product_id`
WHERE `cart`.`u_id` = 1;

Answer 2

You can calculate total cart cost using SUM aggregation function.
SELECT SUM(`cart`.`quantity` * `products`.`product_price`) as `CartTotal`
FROM `cart`
INNER JOIN `products` ON `products`.`product_id` = `cart`.`product_id`
WHERE `cart`.`u_id` = 1;
Also you can use UNION construction for combine cart total result to main query like:
-- Get cart content for user
SELECT
`cart`.`product_id`,
`cart`.`user_id`,
`cart`.`quantity`,
`products`.`product_price` as `unit_price`,
`products`.`product_name`,
`cart`.`quantity` * `products`.`product_price` as `Total`
FROM `cart`
INNER JOIN `products` ON `products`.`product_id` = `cart`.`product_id`
WHERE `cart`.`u_id` = 1
UNION -- Union summary row to cart results
SELECT
NULL as `product_id`, -- set NULL for non aggregated field
`cart`.`user_id`,
COUNT(*), -- calculate items count in cart
NULL as `unit_price`, -- set NULL for non aggregated field
'Total' as `product_name`, -- set 'Total' string as product name
SUM(`cart`.`quantity` * `products`.`product_price`) as `CartTotal` -- calculate total cart cost
FROM `cart`
INNER JOIN `products` ON `products`.`product_id` = `cart`.`product_id`
WHERE `cart`.`u_id` = 1;