当使用带有多个数据库表语句的INNER JOIN时，什么意思/发生？ [英] What means / happens when using the `INNER JOIN` with multiple database table statements?

问题描述

我使用Ruby on Rails 3.2.2和MySQL。我有一个方法（ has_many：通过 ActiveRecord :: Associations ）生成以下SLQ查询：

I am using Ruby on Rails 3.2.2 and MySQL. I have a method (an has_many :through ActiveRecord::Associations) that generates the following SLQ query:

SELECT DISTINCT `articles`.*
           FROM `articles`
     INNER JOIN `articles_comments_associations` `comment_associations_articles`
             ON `comment_associations_articles`.`article_id` = `articles`.`id`
     INNER JOIN `articles_comments_associations`
             ON `articles`.`id` = `articles_comments_associations`.`article_id`
          WHERE `articles_comments_associations`.`comment_id` = 223
            AND (articles_comments_associations.user_id IN (2))

$ b b

我想理解它的含义 INNER JOIN'articles_comments_associations''comment_associations_articles'（注意：有多个数据库表语句 INNER JOIN ）以及SQL查询的工作原理，因为我没有名为的数据库表 $ c> 。 是错误（即使它按预期工作）？

I would like to understand what it means INNER JOIN 'articles_comments_associations' 'comment_associations_articles' (note: there are multiple database table statements for the INNER JOIN) and how it is possible that the SQL query works since I do not have a database table named comment_associations_articles. Is it an error (even if it works as expected)?

推荐答案

p>它是一个表别名。意思是，为了在查询中进一步引用，将表 articles_comments_associations 重命名为 comment_associations_articles 。任何字段或表都可以通过简单地在表/字段引用之后提供另一个名称来进行别名。

It is a table alias. Meaning, it is renaming the table articles_comments_associations to comment_associations_articles for the purpose of further references in the query. Any field or table can be aliased by simply giving another name following the table/field reference.

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