mysql - MySQL：当电子邮件有多个附件时防止重复行

Question

这是我的 MySQL 查询，用于显示文件夹中的电子邮件（当然是瘦身）...

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，如果一封电子邮件有多个附件，同一封电子邮件将出现两次，例如email_message带有id4 的电子邮件。

在不更改 SQL 表结构的情况下，如何返回任何值（例如 1）来表示电子邮件有附件而不返回重复行？

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从句

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