mysql - 如何获取分组行的 ID？

Question

我有一张这样的表:

// notifications
+----+--------+-----------+---------+--------------------+
| id | money  | post_id   | user_id | belongs_to_user_id |
+----+--------+-----------+---------+--------------------+
| 1  | 5      | 1         | 123     | 101                |
| 2  | 10     | 2         | 123     | 101                |
| 3  | -2     | 4         | 456     | 101                |
| 5  | -2     | 2         | 456     | 101                |
| 6  | -2     | 3         | 123     | 101                |
| 7  | 5      | 4         | 789     | 101                |
| 8  | 10     | 4         | 789     | 101                |
+----+--------+-----------+---------+--------------------+

这是我的查询:

SELECT * FROM notifications
WHERE belongs_to_user_id = 101
GROUP BY post_id, user_id
ORDER BY id DESC
LIMIT 3

电流输出 应该是这样的:

+----+--------+-----------+---------+--------------------+
| 5  | -2     | 2         | 456     | 101                |
| 6  | -2     | 3         | 123     | 101                |
| 8  | 10     | 4         | 789     | 101                |
+----+--------+-----------+---------+--------------------+

第七行是分组的，我们在结果中看不到它。这正是问题所在。这是 预期结果 :

+----+--------+-----------+---------+--------------------+
| 5  | -2     | 2         | 456     | 101                |
| 6  | -2     | 3         | 123     | 101                |
| 7  | 5      | 4         | 789     | 101                |
| 8  | 10     | 4         | 789     | 101                |
+----+--------+-----------+---------+--------------------+

如果我删除 GROUP BY ，那么第五个将被省略。所以这是逻辑:

I want to the last three rows (regardless grouping). In other word, Emm ... it's hard to say, I want to select grouped rows (but not counting in LIMIT).

知道我该怎么做吗？

Answer 1

请试试这个查询。它将获得最后三个“组”，然后提取这些组的所有行(使用连接):

SELECT t.*
  FROM notifications t
 INNER JOIN (SELECT s.post_id, s.user_id
               FROM notifications s
              WHERE belongs_to_user_id = 101
              GROUP BY post_id, user_id
              ORDER BY post_id DESC, user_id DESC
              LIMIT 3) u
    ON u.post_id = t.post_id
   AND u.user_id = t.user_id
 WHERE t.belongs_to_user_id = 101
 ORDER BY t.id

更新:在子查询中使用 DISTINCT 的相同查询:

SELECT t.*
  FROM notifications t
 INNER JOIN (SELECT DISTINCT s.post_id, s.user_id
               FROM notifications s
              WHERE belongs_to_user_id = 101
              ORDER BY post_id DESC, user_id DESC
              LIMIT 3) u
    ON u.post_id = t.post_id
   AND u.user_id = t.user_id
 WHERE t.belongs_to_user_id = 101
 ORDER BY t.id