结论: 没有区别, Left Join实际上是Left Outer Join的简写

从逻辑上来讲, Outer Join也就是外连接是需要以两个表中的一个作为参照, 因此才有Left 和 Right之分

而Outer Join的反义词Inner Join也就是内连接是取两个表的交集, 不需要参照, 所以也就不需要区分Left 和 Right

因此, 带Left 或 Right的Join 只能是Outer Join, 对于Inner Join指定Left 或 Right没有实际意义.

 

以下是从这篇博客的评论区引用来的, 原始出处未知:

The keyword OUTER is marked as optional (enclosed in square brackets), and what this means in this case is that whether you specify it or not makes no difference. Note that while the other elements of the join clause is also marked as optional, leaving them out will of course make a difference.

附一张各类Join图解