当此值存储在时间戳类型列中时，如何选择每个组的最大值？

Question

I have the following table in my database.

CREATE TABLE `market_messages` (
  `pm_id` int(11) NOT NULL,
  `pm_item_index` int(11) DEFAULT NULL,
  `pm_item_id` varchar(11) COLLATE utf8_polish_ci NOT NULL,
  `pm_item_sender` varchar(30) COLLATE utf8_polish_ci DEFAULT NULL,
  `pm_item_client` varchar(30) COLLATE utf8_polish_ci NOT NULL,
  `pm_item_sender_id` varchar(30) COLLATE utf8_polish_ci DEFAULT NULL,
  `pm_item_client_id` varchar(30) COLLATE utf8_polish_ci DEFAULT NULL,
  `pm_item_name` varchar(80) COLLATE utf8_polish_ci DEFAULT NULL,
  `pm_sender` varchar(30) COLLATE utf8_polish_ci DEFAULT NULL,
  `pm_receiver` varchar(30) COLLATE utf8_polish_ci DEFAULT NULL,
  `pm_entry_start` timestamp NULL DEFAULT NULL,
  `pm_message` text COLLATE utf8_polish_ci DEFAULT NULL,
  `pm_index` int(11) DEFAULT NULL,
  `pm_type` varchar(10) COLLATE utf8_polish_ci DEFAULT NULL,
  `pm_user_offer` varchar(50) COLLATE utf8_polish_ci DEFAULT NULL,
  `pm_status` varchar(30) COLLATE utf8_polish_ci DEFAULT NULL
) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_polish_ci;

Now I'm trying to group messages and order in specific sequence on the list. I need to list comments based on last entry into m_entry_start column which type is timestamp.

Here is my query, but result is not good:

$sql = "SELECT * 
        FROM market_messages 
        WHERE (pm_item_client = '$user_name' 
                AND pm_item_client_id = '$user_id') 
        OR      (pm_item_sender = '$user_name' 
                AND pm_item_sender_id = '$user_id') 
        GROUP BY pm_item_id, pm_item_client_id, pm_item_sender_id 
        ORDER BY MAX(pm_entry_start) DESC;";      

Result is not this what I have expected. Echo $fetch['m_entry_start']; doesn't give me right result.

I'm expecting max value of pm_entry_start timestamp from each group, but in results is taken probably first value. Additionally `pm_entry_start' is echoed with ago() function to show elapsed time. Ago function is converting timestamp in format 2021-10-01 10:00:19 to something like 3 minutes ago, or 1 day ago etc. This is example result:

$user_name	ago(timestamp)
Adam	1 month ago
Eve	23 days ago
Terese	2 months ago
Mark	5 days ago
Monica	3 min ago

I expecting results in order based on timestamp in descending order so from the last entry to the first.

$user_name	ago(timestamp)
Monica	3 min ago
Mark	5 days ago
Eve	23 days ago
Adam	1 month ago
Terese	2 months ago

How to write the good query? I have started programming in PHP last year. This project is my first with use of MySQL database. Any help appreciated.

Answer 1

Finally I have found the solution: SELECT ..., MAX(pm_entry_start) AS pm_entry_start FROM ... was missing part of MySQL query. Before I have tried only: SELECT ..., MAX(pm_entry_start) FROM ...