SQL:在两个表上联接，其中on列为null [英] SQL: Join on two tables where on column is null

问题描述

我正在使用mysql数据库.我有两个表的错误和配置文件. Bug表有两列(assigned_to，qa_contact)，它们通过多对一关系指向配置文件. 这些是我查询的简化版本.

I am using mysql database. I have two tables bugs and profiles. Bugs table has two columns (assigned_to, qa_contact) pointing to profiles by a many to one relationship. These are the simplified version of my queries.

首先，我尝试执行此操作，但是它会返回重复的行，其中bugs表中的qa_contact为null

Firstly, I was trying to do this but it returns duplicate rows where qa_contact is null in the bugs table

select 
  bug_id, 
  desc, 
  dev.assigned_to, 
  qa.qa_contact 
from 
 bugs, 
 profiles as dev, 
 profiles as qa
where 
  assigned_to = dev.userid 
  and (qa_contact = qa.userid or qa_contact is null)

第二，我的新方法是:

select bug_id, desc, dev.assigned_to, qa.qa_contact 
from 
 bugs, 
 profiles as dev, 
 profiles as qa
where 
  assigned_to = dev.userid 
  and qa_contact = qa.userid

 UNION

select bug_id, desc, dev.assigned_to, null 
from 
 bugs, 
 profiles as dev, 
 profiles as qa
where 
  assigned_to = dev.userid 
  and qa_contact is null

但是在第二种方法中，它排除了qa_contact为null的结果.谁能建议一种有效的方法，因为我正在处理数百万个记录，并且想在结果集上添加更多过滤器.

But in the second approach it excludes the result where qa_contact is null. Can anyone suggest a efficient way of doing this because I am dealing with records in order of millions and would like to add more filters on the resultset.

推荐答案

这是LEFT JOIN的用途:

This is what LEFT JOINs are for:

SELECT bug_id, `desc`, dev.assigned_to, qa.qa_contact 
FROM bugs
INNER JOIN profiles as dev ON bugs.assigned_to = dev.userid
LEFT OUTER JOIN profiles as qa ON bugs.qa_contact = qa.userid

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