JOIN如何在MySQL中工作？ [英] How does JOIN work in MySQL?

问题描述

尽管问题标题与许多讨论重复，但我没有找到这个问题的答案：

Although the question title is duplicate of many discussions, I did not find a answer to this question:

考虑一个简单的标签连接， p>

Consider a simple join for normalized tables of tags as

SELECT tags.tag
FROM tags
    INNER JOIN tag_map
    ON tags.tag_id=tag_map.tag_id
WHERE article_id=xx

JOIN 使用标签和tag_map的整个表，然后过滤创建的（ JOIN ed）表以查找 WHERE 条款for article id

Does JOIN work with the entire tables of tags and tag_map then filter the created (JOINed) table to find rows with WHERE clause for the article id

或 JOIN 只会连接其中article_id = xx的tag_map表的行？

OR JOIN will only join rows of tag_map table in which article_id=xx ?

后一种方法应该会更快！

The latter method should be quite faster!

推荐答案

做前者，根据我的知识，WHERE是在结果JOINed表上显式执行。 （免责声明：MySQL可能会优化这在某些情况下，我不知道）。

It will do the former, to my knowledge WHERE's are explicitly performed on the resulting JOINed table. (Disclaimer: MySQL may optimize this in some cases, I don't know).

要强制后面的行为和执行WHERE第一，你可以添加一个额外的过滤器到您的JOIN ON语句：

To force the latter behaviour and execute the WHERE first, you can add an extra filter to your JOIN ON statement:

SELECT tags.tag 
    FROM tags 
    INNER JOIN tag_map 
        ON tags.article_id=xx
        AND tags.tag_id=tag_map.tag_id 
    WHERE article_id=xx

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