SELECT a column and SUM() of values from another table in SQL - mysql

Question

I'm pretty new with SQL, and this is giving me trouble. The idea is that I have several tables. Here are the relevant tables and columns:
customers:
customer_id, customer_name
orders:
order_id, customer_id
orderline:
order_id, item_id, order_qty
items:
item_id, unit_price
I need to return customer_name as well as total revenue from that customer (calculated as item_price * order_qty * 2).
Here's what I have written:
SELECT customers.customer_name, sum(revenue)
FROM SELECT orderline.order_qty * items.unit_value * 2 AS revenue
FROM orderline
INNER JOIN orders
ON orderline.order_id = orders.order_id
INNER JOIN customers
ON revenue.customer_id = customers.customer_id;
This throws a syntax error and I'm not really sure how to proceed.
This is only one example of this type of problem that I need to work out, so more generalized answers would be helpful.
Thanks in advance!
EDIT:
With help from answers I ended up with this code, which just gets total revenue and puts it next to the first person in the DB's name. What did I get wrong here?
SELECT customers.customer_name, sum(revenue)
FROM(SELECT orderline.order_qty * items.unit_price * 2 AS revenue, orders.customer_id AS CustomerID
FROM( orderline
INNER JOIN orders
ON orderline.order_id = orders.order_id
INNER JOIN items
ON orderline.item_id = items.item_id)) CustomerOrders
INNER JOIN customers
ON CustomerOrders.CustomerID = customers.customer_id;

Answer 1

A couple issues with your query.
First, you need to scope your subquery and alias it:
(SELECT orderline.order_qty * items.unit_value * 2 AS revenue
FROM orderline
INNER JOIN orders
ON orderline.order_id = orders.order_id) CustomerOrders
Secondly, you need to select more than the revenue in the subquery since you are joining it to your customers table
(SELECT
orderline.order_qty * items.unit_value * 2 AS revenue,
orders.customer_id AS CustomerId
FROM
orderline
INNER JOIN orders ON orderline.order_id = orders.order_id) CustomerOrders
Then you need to use the subquery alias in the join to the customers table and wrap it all up in a group by customer_id and CustomerOrders.Revenue
I would tend to do it differently. I'd start with selecting from the customer table, because that is the base of what you are looking for. Then I'd do a cross apply on the orders that would all aggregating the order revenue in the subquery. It would look like this (tsql, you could do the same in mysql with a join with some aggregation):
SELECT
customers.customer_name,
ISNULL(customerOrders.Revenue, 0) AS Revenue
FROM
customers
OUTER APPLY (
SELECT
SUM (orderline.order_qty * items.unit_value * 2) AS Revenue
FROM
orders
INNER JOIN
orderline ON orders.order_id = orderline.order_id
INNER JOIN
items on orderline.item_id = items.item_id
WHERE
orders.customer_id = customers.customer_id
) CustomerOrders
In this case, the subquery aggregates all your orders for you and only returns one row per customer, so no extraneous returned data. Since it's an outer apply, it will also return null for customers with no orders. You could change it to a CROSS APPLY and it will filter out customers with no orders (like an INNER JOIN).

Answer 2

SELECT c.customer_name,
sum(COALESCE(ol.order_qty,0) * COALESCE(i.unit_value,0) * 2)
FROM customers c
INNER JOIN orders o
ON o.customer_id = c.customer_id;
INNER JOIN orderline ol
ON ol.order_id = o.order_id
INNER JOIN items i
ON i.item_id = ol.item_id
GROUP BY c.customer_id

Answer 3

select customer_name, sum(item_price * order_qty * 2) as total_revenue
from (
select * from customers
inner join orders using(customer_id)
inner join orderline using(order_id)
inner join items using(item_id)
)
group by customer_name

Answer 4

select
c.customer_name,
r.revenue
from
customers c
inner join
orders ord on
ord.customer_id = c.customer_id
inner join
(select i.item_id, o.order_id, sum(o.order_qty * items.unit_value * 2) as revenue
from orderline o
inner join items i on
i.item_id = o.item_id
group by o.order_id, i.item_id) as r on r.order_id = o.order_id