MySQL：当电子邮件包含多个附件时，防止重复行

这里是我的MySQL查询，用于显示文件夹中的电子邮件（当然是精简了）

问题是，如果一封电子邮件有多个附件，那么同一封电子邮件将出现两次，例如带有

id

4的

email\u消息

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

add

DISTINCT

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

分组依据

子句

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

---

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

分组依据

子句

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