SELECT attribute with MIN value from set of attribute instances that are part of specific row? - postgresql

Question

I know my question is probably very vague and hard to understand at first glance, and i've sat 30 minutes thinking about a proper title. However my database knowledge is very limited so I have a hard time formulating myself properly yet.
It is part of a school assignment I'm currently doing, where the following is what I'm trying to achieve:
and an ER diagram I made of the system:
What I'm trying to accomplish is, selecting the quantity of the component within the computer_system that has the lowest quantity (current stock) so that I in the dataset I am printing out, am able to state exactly how many of each computer_system the store is able to sell, based on the lowest current quantity of any component the computer_system consists of.
This is the query that i am currently working with to accomplish it, but I've had multiple problems with quantity being ambiguous and other errors every time I try to make a fix. I have consulted a dozen of friends from class, but without luck.
SELECT
computer_system.NAME,
cpu.name as cpu,
gpu.name as gpu,
board.name as mainboard,
pccase.name as pc_case,
ram.name as ram,
component.quantity as qty,
(cpu.price *1.3+ board.price*1.3 + pccase.price*1.3 + ram.price*1.3 + gpu.price*1.3) as computer_system_price
FROM computer_system, component
join component cpu on cpu.id = computer_system.cpu
join component gpu on gpu.id = computer_system.gpu
join component board on board.id = computer_system.mainboard
join component pccase on pccase.id = computer_system.pc_case
join component ram on ram.id = computer_system.ram
JOIN component qty ON qty.quantity = (SELECT MIN(component.quantity) FROM component WHERE component.id IN
(computer_system.pc_case,
computer_system.mainboard,
computer_system.cpu,
computer_system.gpu,
computer_system.ram))

Answer 1

The following code fixed it for me:
SELECT computer_system.name AS "System name",
cpu.name AS "CPU",
gpu.name AS "GPU",
pc_case.name AS "Case",
mainboard.name AS "Mainboard",
ram.name AS "RAM",
FLOOR((cpu.price + mainboard.price + pc_case.price + ram.price + coalesce(gpu.price, 0))*1.3/100)*100+99 AS "System price",
SYSTEM.maxamount
FROM computer_system
join (SELECT name,
id,
price
FROM component) AS cpu
ON cpu.id = computer_system.cpu
left join (SELECT name,
id,
price
FROM component) AS gpu
ON coalesce(gpu.id,0) = computer_system.gpu
join (SELECT name,
id,
price
FROM component) AS pc_case
ON pc_case.id = computer_system.pc_case
join (SELECT name,
id,
price
FROM component) AS mainboard
ON mainboard.id = computer_system.mainboard
join (SELECT name,
id,
price
FROM component) AS ram
ON ram.id = computer_system.ram
join (SELECT computer_system.name,
Min(component.quantity) AS maxamount
FROM computer_system,
component
WHERE computer_system.cpu = component.id
OR computer_system.gpu = component.id
OR computer_system.mainboard = component.id
OR computer_system.pc_case = component.id
OR computer_system.ram = component.id
GROUP BY computer_system.name) AS SYSTEM
ON SYSTEM.name = computer_system.name;