从属于特定行的一组属性实例中选择具有 MIN 值的属性？答案

【问题标题】：SELECT attribute with MIN value from set of attribute instances that are part of specific row?从属于特定行的一组属性实例中选择具有 MIN 值的属性？
【发布时间】：2016-04-05 19:18:11
【问题描述】：

我知道我的问题乍一看可能非常模糊且难以理解，而且我已经坐了 30 分钟来考虑一个合适的标题。但是我的数据库知识非常有限，所以我还很难正确地制定自己的公式。

这是我目前正在做的学校作业的一部分，以下是我想要实现的目标：

以及我制作的系统 ER 图：

我想要完成的是，在计算机系统中选择数量最少的组件的数量（当前库存），以便我在要打印的数据集中能够准确说明每个组件的数量computer_system 商店能够销售的计算机系统，基于计算机系统所包含的任何组件的最低当前数量。

这是我目前正在使用的查询来完成它，但是每次我尝试修复时，我都会遇到数量不明确和其他错误的多个问题。我咨询了十几个班上的朋友，但都没有运气。

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))

【问题讨论】：

标签： postgresql select join

【解决方案1】：

以下代码为我修复了它：

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;

【讨论】：