MySQL 按类别分组，每个类别限制 N 个

Question

TABLE

id       title        category

1        hello1          1
2        hello2          2
3        hello3          1

查询

select  *
from    videos
where   category in
        (
        select  category
        from    videos
        group by 
               category
        having 
               count(*) < 3
ORDER BY RAND()
        )

我的目标是以随机顺序从每个类别中获得 2 个标题我也想以这种方式获取记录

category1
title1
title2

category2
title1
title2

Answer 1

已更新

请尝试这个更新后的查询（SQL Fiddle - http://sqlfiddle.com/#!2/de35bb/9）：

select  videos.*
from    videos
where   
    (
        select  COUNT(vid.id)
        from    videos AS vid
        WHERE videos.category = vid.category
    ) <= 2
ORDER BY
  videos.category, RAND()

我在这里找到了解决方案： Using LIMIT within GROUP BY to get N results per group? 其中答案指向文章 How to select the first/least/max row per group in SQL 作者在其中描述了执行此类任务的一些技术。上面的查询是建立在第二篇文章中提供的示例之上的，但是还有其他形式可以解决这个问题。

Answer 2

下面的查询从每个类别中给出不超过两个随机行：

SELECT title, category
FROM (
  SELECT v.*,
     if( category = @last_cat,
         if( @last_cat:=category, @x:=@x+1,@x:=@x+1),
         if( @last_cat:=category, @x:=0,@x:=0)
     ) x
  FROM (SELECT @last_cat:=-9876, @x:=-91234) x,
       (SELECT * FROM videos ORDER BY category, rand()) v
) x
WHERE x < 2

演示：http://sqlfiddle.com/#!2/59cf9/8

MySQL 按类别分组，每个类别限制 N 个

MySQL Group by category limit N from each category

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