数据库中每个数据的类别由特定的 id 并且只允许为每个类别 mysql 获取 2答案

【问题标题】：categories for each data in database by a specific id and only allow get 2 for each categories mysql数据库中每个数据的类别由特定的 id 并且只允许为每个类别 mysql 获取 2
【发布时间】：2014-12-06 08:44:19
【问题描述】：

可能重复MySql Query: Select top 3 rows from table for each category。我不清楚为什么我又问了一个答案。

这是我的数据库

+-------+----------+-------------+
|  id   |  app_Id  |   Content   |
+-------+----------+-------------+
|   1   |    19    | Hello Peer  |
+-------+----------+-------------+
|   2   |    20    | Hello Peer  |
+-------+----------+-------------+
|   3   |    19    | Hello Peer  |
+-------+----------+-------------+
|   4   |    19    | Hello Peer  |
+-------+----------+-------------+
|   5   |    21    | Hello Peer  |
+-------+----------+-------------+
|   6   |    20    | Hello Peer  |
+-------+----------+-------------+
|   7   |    19    | Hello Peer  |
+-------+----------+-------------+
|   8   |    20    | Hello Peer  |
+-------+----------+-------------+
|   9   |    21    | Hello Peer  |
+-------+----------+-------------+

现在如何按 app_Id 选择记录类别，每个类别限制为 2 并按 appId desc 排序

如果上面的陈述不清楚。这是我预期的上面数据库的输出记录。

+-------+----------+-------------+
|  id   |  app_Id  |   Content   |
+-------+----------+-------------+
|   4   |    19    | Hello Peer  |
+-------+----------+-------------+
|   7   |    19    | Hello Peer  |
+-------+----------+-------------+
|   6   |    20    | Hello Peer  |
+-------+----------+-------------+
|   8   |    20    | Hello Peer  |
+-------+----------+-------------+
|   5   |    21    | Hello Peer  |
+-------+----------+-------------+
|   9   |    21    | Hello Peer  |
+-------+----------+-------------+

我希望查询这样做有什么想法吗？

【问题讨论】：

标签： mysql sql select sql-order-by greatest-n-per-group

【解决方案1】：

试试这个：

SELECT id, app_Id, Content 
FROM (SELECT id, app_Id, Content, 
             IF(@appId=@appId:=app_Id, @rowNo:=@rowNo+1, @rowNo:=1) AS rowNo 
      FROM tableA, (SELECT @rowNo:=1, @appId:=0) a
      ORDER BY app_Id, id DESC
     ) AS a 
WHERE rowNo <= 2
ORDER BY app_Id

【讨论】：

【解决方案2】：

您可以使用更简单的版本获得所需的结果

select c.* from category c
where
(
  select count(*) from category as c1
  where c.app_Id = c1.app_Id
  and c.id < c1.id
)<=1
order by c.app_Id;

【讨论】：