mysql查询问题：获取最新帖子

Question

我有一张用户帖子的表格。

我正在尝试使用此查询从帖子表中获取所有用户的最新帖子（只有 1 个）：

SELECT * FROM Posts WHERE PostedAs = 'USER' GROUP BY PostedAsId ORDER BY
CONCAT(PostedDate,PostedTime) DESC

每个用户只给我 1 个帖子，但问题是，它给我的结果是基于所有帖子的降序排列。

我想获取所有用户的最新帖子。 我做错了什么？

Answer 1

我认为这应该可行，但我不知道它是否足够快。

SELECT * 
FROM Posts p
WHERE PostedAs = 'USER' 
  AND CONCAT(PostedDate, PostedTime) = (SELECT MAX(CONCAT(PostedDate, PostedTime))
                                        FROM Posts p2
                                        WHERE p.PostedAsId = p2.PostedAsId)

Answer 2

$sql = mysql_query("SELECT * FROM Posts WHERE PostedAs = 'USER' GROUP BY PostedAsId ORDER BY
CONCAT(PostedDate,PostedTime) DESC");
while($row = mysql_fetch_array($sql)
{
 echo $row['post'];
}

上面的结果是什么？

Answer 3

假设您有一个日期时间字段，我们称之为PostedTimestamp，您可以这样进行查询：

SELECT * FROM Posts
INNER JOIN
    (SELECT
      PostedAsID,
      MAX(PostedTimestamp) AS latestPostTimestamp
    FROM Posts) AS LatestPosts
  ON Posts.PostedAsID = LatestPosts.PostedAsID
WHERE Posts.PostedAs = 'USER'

Answer 4

我已经复制了您在最初问题中描述的内容，但我看不出原始 concat 查询存在什么问题。

表格如下：-

mysql> desc posts;
+------------+------------+------+-----+------------+----------------+
| Field      | Type       | Null | Key | Default    | Extra          |
+------------+------------+------+-----+------------+----------------+
| PostId     | int(11)    | NO   | PRI | NULL       | auto_increment |
| PostedAs   | varchar(8) | YES  |     | USER       |                |
| PostedAsId | int(11)    | YES  |     | NULL       |                |
| PostedDate | date       | YES  |     | 0000-00-00 |                |
| PostedTime | time       | YES  |     | 00:00:00   |                |
+------------+------------+------+-----+------------+----------------+
5 rows in set (0.01 sec)

表中的数据为：-

mysql> select * from posts;
+--------+----------+------------+------------+------------+
| PostId | PostedAs | PostedAsId | PostedDate | PostedTime |
+--------+----------+------------+------------+------------+
|      1 | USER     |         22 | 1977-01-13 | 12:34:56   |
|      2 | USER     |         47 | 1977-02-13 | 00:00:00   |
|      3 | USER     |         52 | 1977-03-13 | 00:00:00   |
|      4 | USER     |          7 | 1977-04-13 | 00:00:00   |
|      5 | USER     |         47 | 1977-05-13 | 00:00:00   |
|      6 | USER     |         22 | 1977-06-13 | 00:00:00   |
|      7 | USER     |         52 | 1977-07-13 | 00:00:00   |
|      8 | USER     |         52 | 1977-08-13 | 00:00:00   |
|      9 | USER     |         59 | 1977-09-13 | 00:00:00   |
|     10 | USER     |          7 | 1977-10-13 | 00:00:00   |
|     11 | USER     |          7 | 1977-11-13 | 00:00:00   |
+--------+----------+------------+------------+------------+
11 rows in set (0.00 sec)

运行问题中提供的查询

 mysql> SELECT * FROM Posts
        -> WHERE PostedAs = 'USER'
        -> GROUP BY PostedAsId
        -> ORDER BY CONCAT(PostedDate,PostedTime) DESC;
    +--------+----------+------------+------------+------------+
    | PostId | PostedAs | PostedAsId | PostedDate | PostedTime |
    +--------+----------+------------+------------+------------+
    |      9 | USER     |         59 | 1977-09-13 | 00:00:00   |
    |      4 | USER     |          7 | 1977-04-13 | 00:00:00   |
    |      3 | USER     |         52 | 1977-03-13 | 00:00:00   |
    |      2 | USER     |         47 | 1977-02-13 | 00:00:00   |
    |      1 | USER     |         22 | 1977-01-13 | 12:34:56   |
    +--------+----------+------------+------------+------------+
    5 rows in set (0.00 sec)

同一查询的重新排列（排序）：-

mysql> SELECT * FROM Posts
    -> WHERE PostedAs = 'USER'
    -> GROUP BY PostedAsId
    -> ORDER BY PostedDate DESC, PostedTime DESC;
+--------+----------+------------+------------+------------+
| PostId | PostedAs | PostedAsId | PostedDate | PostedTime |
+--------+----------+------------+------------+------------+
|      9 | USER     |         59 | 1977-09-13 | 00:00:00   |
|      4 | USER     |          7 | 1977-04-13 | 00:00:00   |
|      3 | USER     |         52 | 1977-03-13 | 00:00:00   |
|      2 | USER     |         47 | 1977-02-13 | 00:00:00   |
|      1 | USER     |         22 | 1977-01-13 | 12:34:56   |
+--------+----------+------------+------------+------------+
5 rows in set (0.00 sec)

使用您自己提供的 concat order by 或我自己的 double order by 似乎都可以为上面详述的表格/数据提供正确的结果，每个用户都被选中一次，发布日期/ID 是最新发布的.

如果您的架构与我在此处提供的架构存在显着差异，请随时澄清。

从表面上看，这个问题似乎在原来的问题中有解决方案。

祝你好运！