mysql group by uid 哪个 uid 匹配另一个查询答案

【问题标题】：mysql group by uid which uid match another querymysql group by uid 哪个 uid 匹配另一个查询
【发布时间】：2012-03-04 12:28:19
【问题描述】：

我想得到 10 兰特的结果，image !=''、uid 分组，这些uid 与select uid from user_table where type='1' 相同。但我的查询只返回 2 个结果。问题出在哪里？

select * from article_table where image!='' 
order by rand() 
group by uid 
in (select uid from user_table where type='1') 
limit 10

【问题讨论】：

标签： mysql group-by

【解决方案1】：

我会用 join 代替

select *
  from article_table at
  join ( select uid
           from user_table
          where type = '1' ) ut
    on at.uid = ut.uid
 where image != ''
 group by at.uid
 order by rand()
 limit 10

或者您可能希望限制user_table 中uids 的数量，以便更快地开始：

select at.*
  from article_table at
  join ( select uid
           from user_table
          where type = '1'
          order by rand()
          limit 10 ) ut
    on at.uid = ut.uid
 where image != ''
 group by at.uid
 order by rand()
 limit 10

我在这里假设每个用户都有很多文章。虽然看起来更可怕，但内部选择中的order by rand() 覆盖了较小的数据集，这会加快速度，而外部选择中的order by 只需处理较少的行数。

请注意，按随机值排序可能会严重影响性能，因为您必须遍历与 where 子句匹配的整个表。有alternatives。

【讨论】：

【解决方案2】：

下面的查询会做到这一点，

SELECT * 
FROM   article_table 
WHERE  image!='' 
       AND uid IN (SELECT DISTINCT uid 
                   FROM   user_table 
                   WHERE  TYPE = '1' 
                   LIMIT  10) 
GROUP  BY uid 
ORDER  BY Rand() 
LIMIT  10

【讨论】：