oracle sql中的join关键字和inner join关键字之间有什么区别? [英] what is the difference between join keyword and inner join keyword in oracle sql?

问题描述

我找不到关键词join上的文档，但是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.

我正在Oracle hr模式中进行一些实验，其中有表departments:

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

• deparment_name
• manager_id
• location_id

• deparment_name
• manager_id
• location_id

表employees:

• first_name
• employee_id

• first_name
• employee_id

和表locations:

• location_id
• city

• location_id
• city

查询应返回部门名称，部门经理的名字以及部门所在的城市.

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inner join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

这两个条件之间是否存在差异，还是我偶然发现了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

• 按照1992 ANSI SQL参考， INNER是可选的 :
• Following 1992 ANSI SQL reference, INNER is optional:

查询表达式179 7.5-联接表

Query expressions 179 7.5 - joined table

3)如果指定了合格的连接，但未指定连接类型 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 Standards( 9i及更高版本)，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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