sql - 在 oracle 10g 中加入查询

Question

我编写了这个查询来显示在多伦多工作的所有员工的姓氏、部门编号和部门名称。

select last_name, job_id, department_id, department_name
from employees e
 join departments d on d.department_id=e.department_id
 join locations l on d.location_id=l.location_id and l.city='Toronto';

我收到此错误 ORA-00918: column ambiguously defined

Answer 1

当参与连接的两个表上都存在列时，您需要在列名前加上别名，以指定您想要的列。在您的联接中department_id，两个表共享，您可以指定要d.department_id在选定列列表中使用的列。

select 
  last_name, 
  job_id, 
  d.department_id, --specify which table you want this ambiguous column from
  department_name
from employees e
  join departments d 
  on d.department_id=e.department_id
  join locations l on 
  d.location_id=l.location_id and l.city='Toronto';

Answer 2

使用别名从任何特定表中选择列。例如，简单地写department_id会引发错误，因为它在多个表上可用并且会引发歧义错误

因此，更好的解决方案是选择具有别名的列，例如

 select e.last_name, e.job_id, e.department_id, d.department_name
 from employees e
 join departments d on d.department_id=e.department_id
 join locations l on d.location_id=l.location_id and l.city='Toronto';

Answer 3

将第一行更改为：

select e.last_name, e.job_id, e.department_id, d.department_name

Answer 4

尝试这个：

select last_name, job_id, department_id, department_name 
from employees 
join departments using(department_id) 
join locations using(location_id) where city='Toronto';