Question

我有表comments的这些列：

id
content
add_date
uid
school_id

行可以具有相同的school_id。

我想根据add_date选择最新数据，但每个school_id仅 1行（school_id不重复），限制为10。

我已经尝试了很多代码，但它对我不起作用。任何帮助将不胜感激。

Answer 1

这就是我们所说的Greatest N per Group。您可以通过放入子查询来实现此目的，以便可以将其与未分组的表（comments）连接起来。试试这个：

SELECT  c.*
FROM
        (
            SELECT school_id, MAX(add_date) maxDate
            FROM comments
            GROUP BY school_id
        ) x INNER JOIN comments c
            ON  x.school_id = c.school_ID AND
                x.maxDate = c.add_date
ORDER BY x.maxDate desc
LIMIT 10

Answer 2

select C.ID, C.Content, t1.MaxDate as [add_date], C.uid, t1.school_id
from (selet school_id, max(add_Date) as 'MaxDate'
from comments
group by school_id) T1
inner join comments C on T1.school_id = C.school_id and C.add_Date= T1.MaxDate
LIMIT 10

如果您想选择返回10 rows，请添加order by或Where条款

Answer 3

select c1.*
  from comments c1
 where add_date = (select max(add_date) from comments c2 where c2.school_id =c1.school_id)
 order by add_date desc
 limit 10

在评论（add_date）和评论（school_id，add_date）

上创建索引

在mysql中选择具有唯一字段值的行

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