MySQL GROUP BY，同时按列内容保留某些行

Posted 2023-05-09

【中文标题】MySQL GROUP BY，同时按列内容保留某些行

【英文标题】：MySQL GROUP BY while keeping certain rows by column content

【发布时间】：2019-11-25 22:09:55

【问题描述】：

存在重复行的表。请参阅第 1 行和第 2 行：

id   full_name    email           status   active
1    John Doe     [e-mail]   ok       1
2    John Doe     [e-mail]   null     1
3    Ricky Duke   [e-mail]   null     1
4    Jane Doe     [e-mail]   block    1

我需要选择不同的行，而不是随机选择 - 一个不同的行，但要选择具有 'status' NOT NULL 的行。

我的查询是：

SELECT full_name, email
FROM `subscribers`
WHERE active = 1 AND (status = 'ok' OR status IS NULL)
GROUP BY email

该查询随机选择不同的行，而不优先考虑“状态”字段。

我如何优先选择具有 'status' NOT NULL 的不同行，并且仅在不存在“ok”状态的行的情况下才选择具有 NULL 的行？ p>

【问题讨论】：

见Why should I provide an MCRE for what seems to me to be a very simple SQL query

【参考方案1】：

你可以使用row_number():

select s.*
from (select s.*,
             row_number() over (partition by email order by (status is not null) desc) as seqnum
      from subscribers s
      where active = 1
     ) s
where seqnum = 1;

【参考方案2】：

您可以使用执行条件排序的相关子查询进行过滤，并为null 状态提供最低优先级：

select t.*
from mytable t
where t.id = (
    select id
    from mytable t1
    where 
        t1.full_name = t.full_name
        and t1.email = t.email
        and t1.active = t.active
    order by status is null, status  
    limit 1
)

这将重复定义为具有相同 full_name、email 和 active 的记录。您可能希望根据您对重复项的实际定义进行调整。

Demo on DB Fiddle：

编号 |全名 |电子邮件 |状态 |积极的 -: | :--------- | :------------ | :----- | :----- 1 |约翰·多伊 | [e-mail] |好的 | 1 3 |瑞奇·杜克 | [e-mail] | 空 | 1 4 |简·多伊 |简@mail.com |块 | 1

【参考方案3】：

(SELECT full_name, email
FROM `subscribers`
WHERE active = 1 AND status IS NOT NULL
GROUP BY email) 
UNION ALL
(SELECT full_name, email
FROM `subscribers`
WHERE active = 1 AND status IS NULL AND
email not in (SELECT distinct email
FROM `subscribers` 
WHERE active = 1 AND status IS NOT NULL)
GROUP BY email);