sql - 选择多个列

Question

我有这些表：

• 机器
• machine_component_lookup
• 零件

machine_component_lookup 表具有给组件表记录提供键的记录，给定机器表的主键。我想列出组件和机器表中的列

select port,
       portrole,
       machine.machine_key 
  from component a 
 where a.component_key in (select b.component_key 
                             from machine_component_lookup b 
                             join machine c 
                               on b.machine_key =c.machine_key );

我收到错误，因为缺少 table 的 FROM 子句条目，我缺少什么？

Answer 1

数据库引擎在主查询中看不到machine表。如果您想machine_key从表格中显示，请尝试以下代码。

select a.port,
       a.portrole,
       c.machine_key 
  from component a 
  join machine_component_lookup b on  a.component_key = b.component_key 
  join machine c on b.machine_key =c.machine_key ;

Answer 2

How about something like

select a.port,
       a.portrole,
       c.machine_key 
  from component a INNER JOIN
  machine_component_lookup b ON a.component_key = b.component_key INNER JOIN
  machine c  on b.machine_key =c.machine_key

Answer 3

我认为是：

select port, portrole, a.component_key as machine_key from component a 
where a.component_key in (select b.component_key from machine_component_lookup b join machine c on b.machine_key =c.machine_key );