这是一个脏/不符合左连接？ [英] is this a dirty / not conform left join?

问题描述

我需要为表格TBL_B中的每条记录处理某事，以获取来自例如另一个查询的每个结果行。 TBL_A：

I need to process "something" for each record in a table TBL_B for every result line from another query from e.g. TBL_A:

SELECT
  *
FROM TBL_A
LEFT JOIN TBL_B ON '1' = '1'   // is this really clean??
WHERE TBL_A.ID <does match a condition>

这两个表没有任何主 - 外关系。一切正常。我唯一担心的是，标记为LEFT JOIN（与interbase一起使用）的粗体仅仅是偶然的。



我的问题：这种LEFT JOIN SQL是否兼容？



提前谢谢。

布鲁诺



NB 0：相反左连接我尝试了FROM TBL_A，TBL_B，这没有达到预期的结果（这对我来说不清楚为什么不......）



NB 1：是的，我知道，SELECT *并不好。我写这篇文章只是为了让问题简短;）



N.B. 2：将Q& A列表中的主题与您在打开问题时看到的内容进行比较。工作中的偏执狂程序员* lol *

The two tables do not have any Primary-Foreign relation. Everything works fine. My only concern is, that the bold marked LEFT JOIN (which works with interbase) does only work by chance.

My question: Is this kind of LEFT JOIN SQL compliant?

Thank you in advance.
Bruno

N.B. 0: Instead left join I tried "FROM TBL_A,TBL_B" which did not result in the expected result (and which is not clear for me why not....)

N.B. 1: And yes I know, SELECT * is not good. I wrote it only to make the question short ;)

N.B. 2: Compare subject in Q&A list against what you see when you open the question. Paranoia programmers at work *lol*

推荐答案

因为'1'='1'是一个完全有效的where子句，这是一个有效的LEFT JOIN并且有效。

实际上这是写一个CROSS JOIN的一种方式......

http://www.w3resource.com/sql/joins/cross-join.php [ ^ ]

As '1' = '1' is a perfectly valid where clause, this is a valid LEFT JOIN and works.
In fact this is one way to write a CROSS JOIN...
http://www.w3resource.com/sql/joins/cross-join.php[^]

Bruno，请看这里： SQL连接的可视化表示 [ ^ ]。我想它可能会帮助你理解连接是如何工作的。



Kornfeld是对的，它的意思是交叉加入 [ ^ ]，因为使用 1 = 1 类似于：

Bruno, please have a look here: Visual Representation of SQL Joins[^]. I think it might help you to understand how joins works.

Kornfeld is right, it means cross join[^], because using 1=1 is similar to:

SELECT t1.*, t2.*
FROM t1, t2

这意味着参与加入的表格的笛卡尔积。



LEFT，RIGHT或INNER JOIN意味着：

which means Cartesian product of the tables involved in the join.

LEFT, RIGHT or INNER JOIN means this:

WHERE t1.PK = t2.FK

顺便说一句 1 = 1 总是返回 true 。这是提供sql注入攻击的最简单方法;）



你可以通过这篇文章解决一些挫折：

为什么使用WHERE 1或WHERE 1 = 1？ [ ^ ]

加入1 = 1与交叉加入 [ ^ ]



我不知道怎么解释得更好。如果您有任何其他问题......

By The Way 1=1 always returns true. It's the simplest way to provide sql injection attack ;)

Some frustration you can solve with this articles:
Why use WHERE 1 or WHERE 1=1?[^]
Join on 1=1 versus cross join[^]

I don't know how to explain it better. If you have any other question...

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