仅从MySQL中数据集的最新创建记录中选择所有字段？

Question

I am doing a comments table, and I can't quite figure out this query. These comments will be under a particular data record in this application, which is what comment_number refers to. When a comment is created, it is assigned that records 'comment number'.. The comment number + date_added identify each particular comment. If a comment is updated/edited, instead of updating the record itself, it creates another entry in the table with the same comment_number and date_added, and creates a new date_modified. This is due to compliance issues. I have to be able to go back in that particular comment's history.

Here is the table description:

+----------------+---------------+------+-----+---------+-------+
| Field          | Type          | Null | Key | Default | Extra |
+--------------- +---------------+------+-----+---------+-------+
| comment_number | varchar(64)   | YES  |     | NULL    |       |
| comment        | varchar(2048) | YES  |     | NULL    |       |
| date_added     | datetime      | YES  |     | NULL    |       |
| date_modified  | datetime      | YES  |     | NULL    |       |
| is_deleted     | tinyint(1)    | YES  |     | NULL    |       |
| date_deleted   | datetime      | YES  |     | NULL    |       | 
+--------------- +---------------+------+-----+---------+-------+

So different comments will have the same comment number, but will have different 'date_added' fields. And a particular comment's different versions in history will have the same 'comment_number' and 'date_added' value, but different 'date_modified' fields.

So i need to pull all the fields of the latest modified versions of all comments. I have been playing around with MAX() functions, as well as GROUP BYs, but i have not been able to get the correct results yet.

Any help would be greatly appreciated.

Answer 1

您可以在查询中使用ORDER BY date_modified，然后将LIMIT设置为1。确定是否需要ASC（从低到高）或DESC（从高到低）来获取所需字段作为第一个结果。

Answer 2

To get the most recent date_modified for every comment, you need a query like this:

SELECT
  comment_number,
  date_added,
  MAX(date_edited) AS max_date_edited
FROM
  tablename
GROUP BY
  comment_number,
  date_added

to get the latest version of the comment, you then need to join the result of this query back:

SELECT c.*
FROM
  table_name c INNER JOIN (
    SELECT
      comment_number,
      date_added,
      MAX(date_edited) AS max_date_edited
    FROM
      tablename
    GROUP BY
      comment_number,
      date_added
  ) m ON c.comment_number=m.comment_number
      AND c.date_edited = m.max_date_edited