如何从MYSQL中的组中查找前N个记录的查询结果

Question

我有以下查询，它以desc顺序给出按用户ID分组的名称计数。到目前为止我已经达到了，但不能超越它。我想连接每个用户的前2条记录的名称列。

到目前为止的查询是：

SELECT t.*, 
       IF(@grp = t.user_id, @rowno := @rowno + 1, @rowno := 1) AS rowno, 
       @grp := t.user_id AS u_id 
FROM   (SELECT notes.user_id, 
               t.name        name, 
               Count(t.name) ct 
        FROM   notes 
               INNER JOIN tags t 
                       ON notes.id = t.note_id 
        GROUP  BY notes.user_id, 
                  t.name 
        ORDER  BY notes.user_id, 
                  Count(t.name) DESC) t; 

它给出了以下结果：

+---------+------------+----+-------+-----+
| user_id | name       | ct | rowno | uid |
+---------+------------+----+-------+-----+
| 282     | realifex   | 1  | 1     | 282 |
+---------+------------+----+-------+-----+
| 282     | clear      | 1  | 2     | 282 |
+---------+------------+----+-------+-----+
| 282     | thinking   | 1  | 3     | 282 |
+---------+------------+----+-------+-----+
| 282     | refreshing | 1  | 4     | 282 |
+---------+------------+----+-------+-----+
| 285     | solid      | 2  | 1     | 285 |
+---------+------------+----+-------+-----+
| 285     | clear      | 1  | 2     | 285 |
+---------+------------+----+-------+-----+
| 285     | thinking   | 1  | 3     | 285 |
+---------+------------+----+-------+-----+
| 287     | holidays   | 3  | 1     | 287 |
+---------+------------+----+-------+-----+
| 287     | Larry      | 3  | 2     | 287 |
+---------+------------+----+-------+-----+
| 287     | travel     | 2  | 3     | 287 |
+---------+------------+----+-------+-----+
| 287     | thinking   | 1  | 4     | 287 |
+---------+------------+----+-------+-----+

我试图将前2个结果连接到每个用户组的一列，如下所示：

+---------+----------------+
| user_id | name           |
+---------+----------------+
| 282     | realifex,clear |
+---------+----------------+
| 285     | solid,   clear |
+---------+----------------+
| 287     | Larry,travel   |
+---------+----------------+

Answer 1

Ue Kua zxsw Poi：

group_concat()

注意：

• SELECT group_id, group_concat(name order by rn) as names
  FROM (SELECT t.*, 
               (@rn := IF(@grp = t.user_id, @rowno := @rowno + 1, 
                          IF(@grp := t.user_id, 1, 1)
                         )
               )  as rn
        FROM (SELECT n.user_id, t.name, n.name,  Count(t.name) ct 
              FROM notes n INNER JOIN
                   tags t 
                   ON notes.id = t.note_id 
              GROUP BY n.user_id,  t.name 
              ORDER BY n.user_id, Count(t.name) DESC
             ) t CROSS JOIN
             (SELECT @rn := 0, @grp := -1) params
       ) t
  WHERE rn <= 2
  GROUP BY user_id;

  和@rn的表达式是单个表达式。 MySQL不保证@grp中表达式的评估顺序，因此单个表达式是安全分配这两个变量的唯一方法。
• 变量已初始化。
• SELECT条款是确定“前2”的地方。