Question

Here’s my table structure:

CREATE TABLE USER ( 
  id INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
  username VARCHAR(50),
  `status` VARCHAR(50),
  `dateupdated` DATETIME
) ENGINE = INNODB;  

If the data, has an ID as primary key, then the username can be repeated, let’s say I have 3 users, and all of them had a status update yesterday and today. Now my question is, how to select only the latest status updates from each user ?, here’s my solution

   SELECT user.`username`, latest.status AS latest_status , user.`dateupdated` 
     FROM USER 
LEFT JOIN USER AS latest ON user.`id` = latest.id 
 ORDER BY dateupdated DESC;

…but it seem wrong, because it “also” printed out the data dated yesterday, so how to do that in order to get the exact latest data only for each users and avoid printing out the past datas?

Answer 1

You’re almost there, but to get the latest per user you have to add an extra sorting condition in your JOIN, and a where condition:

SELECT  u.username, u.status AS latest_status , u.dateupdated 
FROM USER u
LEFT JOIN USER u2
 ON u.username=u2.username 
AND u.dateupdated < u2.dateupdated  -- see extra JOIN cond?
WHERE u2.dateupdated IS NULL        -- see extra WHERE cond?
ORDER BY dateupdated DESC;

What this does is join USER to itself not only within username, but such that u‘s dateupdated is less than u2s. Since this a LEFT JOIN, if there is a u.dateupdated such that there is no greater u2.updated for the same username, then the u2.updated is set to NULL.

But these u.dateupdated are precisely the maximum dateupdated for that user, and hence we add the WHERE condition in to grab these rows.