这个 SQL 语句“WHERE table_a.key (+)= table_b.key"是什么意思?方法? [英] What does this SQL statement "WHERE table_a.key (+)= table_b.key" means?

问题描述

我正在阅读学校的一些讲义，我看到了这张幻灯片:

I am reading through some lecture notes of my school, and I come across this slide:

章节:数据集成和 ETL 流程

Chapter: Data Integration and ETL Process

幻灯片标题:重复值问题

文本:重复值总是存在.消除可能很耗时，尽管这是一项简单的任务.

Text: Duplicate values invariably exist. Eliminating can be time consuming, although it is a simple task to perform.

SQL 示例:

SELECT ...
FROM table_a, table_b
WHERE table_a.key (+)= table_b.key
UNION
SELECT ...
FROM table_a, table_b
WHERE table_a.key = table_b.key (+);

具体来说，我不明白(+)=和最后一个(+)的含义.

Specifically, I do not understand the meaning of the (+)= and the last (+).

感谢您的帮助！

推荐答案

这意味着您应该停止使用带有 where 子句中的条件的旧式连接，并且 always使用显式 join 语法.

It means you should stop using old-style joins with the condition in the where clause and always use explicit join syntax.

对于您的特定查询，等效于 full outer join:

For your particular query, the equivalent is full outer join:

SELECT ...
FROM table_a FULL OUTER JOIN
     table_b
     ON table_a.key = table_b.key;

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