Question

我有Category和Question一对多关系表，我想有3个随机类别，每个类别有3个随机问题。如何在一个查询中执行此操作？

Category
id
title

Question
id
title
cat_id

示例输出：

 cat_id title ques_id title cat_id
 4      Math  1       1+1?   1
 4      Math  3       2*5?   1
 4      Math  5       3*5?   1
 12     Hist  1       Who.   12
 12     Hist  2       blah   12
 12     Hist  5       blah   12
 15     Phys  1       m=.    15
 15     Phys  3       blah   15
 15     Phys  4       blah   15

Answer 1

如果您可以将答案分为3行而不是9行，最简单的方法是：

select c.*,
       substring_index(group_concat(q.id order by rand()), ',', 3) as question_ids
from category c join
     question q
     on c.id = q.cat_id
group by c.id
order by rand();

否则，您可以使用变量执行此操作：

select cq.*
from (select c.*, q.*,
             (@rn := if(@c = c.id, @rn + 1,
                        if(@c := c.id, 1, 1)
                       )
             ) as rn
      from (select c.*
            from category c
            order by rand()
            limit 3
           ) c join
           question q
           on c.id = q.cat_id cross join
           (select @c := 0, @rn := 0) params
      order by c.id, rand()
     ) cq
where rn <= 3;

当然，您应该选择实际需要的列，而不是使用*。

MySQL Limit连接表

1 个答案: