MySQL：当电子邮件有多个附件时，防止重复行

Question

这是我用于在一个文件夹中显示电子邮件的MySQL查询（当然是精简的）......

SELECT em.id
FROM email_messages AS em 
LEFT JOIN email_attachments AS ea on ea.id_email=em.id 
WHERE em.id='4'

问题是，如果电子邮件包含多个附件，则相同的电子邮件会出现两次，例如email_message和id 4。

如果不更改SQL表的结构，如何返回任何值（例如1）来表示电子邮件具有附件而不返回重复的行？

Answer 1

添加DISTINCT以仅获取唯一值

SELECT DISTINCT em.id
FROM   email_messages AS em 
       LEFT JOIN email_attachments AS ea 
          ON ea.id_email = em.id 
WHERE  em.id = '4'

或GROUP BY条款

SELECT em.id
FROM   email_messages AS em 
       LEFT JOIN email_attachments AS ea 
          ON ea.id_email = em.id 
WHERE  em.id = '4'
GROUP  BY em.id