JOIN 与多个 FROM 表 [英] JOIN vs Multiple FROM Tables

问题描述

是否有过连接不会返回具有相同条件的多个表返回的数据的情况?

Is there ever a case where a join will not return data that a FROM multiple tables with the same conditions returns?

例如

SELECT *
FROM TableNames as Names
INNER JOIN TableNumbers as Numbers on Names.ID = Numbers.ID

VS

SELECT *
FROM TableNames as Names, TableNumbers as Numbers
WHERE Names.ID = Numbers.ID

推荐答案

INNER JOIN(如您的第一个示例中所示)将始终返回与使用相同连接条件的 WHERE 过滤器的笛卡尔连接相同的数据(您的第二个例子).

An INNER JOIN (as in your first example) will always return the same data as your a cartesian join with a WHERE filter that uses the same join criteria (your second example).

但是，请注意，对于 OUTER JOIN，情况并非如此，其中 NULL 值在笛卡尔连接中被过滤掉，并使用 WHERE 过滤器作为连接条件.

However, note that this is not true for OUTER JOINs, where NULL values are filtered out in a cartesian join with a WHERE filter as join criteria.

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