问题描述

我找不到关于关键字 join 的文档，但我在网上看到了使用它的示例.

I can't find documentations on the key word join but I saw examples on the web using it.

我在 Oracle hr 模式中做了一些实验，我有表 departments:

I was doing some experiment with it in Oracle hr schema, where I have table departments:

• deparment_name
• manager_id
• location_id

表employees:

• first_name
• employee_id

和表locations:

• location_id
• city

查询应返回部门名称，部门经理的名字，部门所在城市.

Query should return the department_name, first_name of the manager of the department, and the city where the department is located.

与使用关键字inner join

带有join的代码:

select d.department_name, e.first_name,l.city
from departments d
   join employees e on d.manager_id=e.employee_id
   join locations l on d.location_id=l.location_id

带有内连接的代码:

select d.department_name, e.first_name,l.city
from departments d
   inner join employees e on d.manager_id=e.employee_id
   inner join locations l on d.location_id=l.location_id

这两种情况是否有区别，或者我只是偶然发现它们返回相同结果的情况?

Is there a difference between the two condition, or am I just happen to stumble on a situation where they return the same results?

推荐答案

• 遵循 1992 ANSI SQL 参考，INNER 是可选的:

查询表达式 179 7.5 - 连接表

Query expressions 179 7.5 - joined table

3) 如果指定了合格的联接而联接类型未指定指定，则 INNER 是隐式的.

3) If a qualified join is specified and a join type is not specified, then INNER is implicit.

• 遵循 Oracle 标准(9i 以后)，INNER 前缀也是可选的.在 9i 之前，Oracle 不遵循 ANSI 规则，甚至不支持 JOIN 语法.
• Following Oracle Standards (9i onward), the INNER prefix is also optional. Before 9i, Oracle didn't follow ANSI rules, and didn't even support JOIN syntax.

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