mysql - Best and optimal way to join max value from other table

Question

I want to find the optimal way to run this query, here the details, lets say the following are my tables with their data:

-- User Table
DROP TABLE IF EXISTS `users`;
CREATE TABLE `users` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(30) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM;

-- Entries
INSERT INTO `users` VALUES (1,'user_a');
INSERT INTO `users` VALUES (2,'user_b');
INSERT INTO `users` VALUES (3,'user_c');

-- User Log Table
DROP TABLE IF EXISTS `user_log`;
CREATE TABLE `user_log` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `idrel` int(11) NOT NULL,
  `event` varchar(20) NOT NULL,
  `log` datetime NOT NULL,
  PRIMARY KEY (`id`),
  KEY `idrel` (`idrel`,`log`)
) ENGINE=MyISAM;

-- Entries
INSERT INTO `user_log` VALUES (1,1,'log1','2014-01-01 13:00:00');
INSERT INTO `user_log` VALUES (2,1,'log2','2014-01-02 13:00:00');
INSERT INTO `user_log` VALUES (3,2,'log3','2014-01-03 13:00:00');
INSERT INTO `user_log` VALUES (4,2,'log4','2014-01-04 13:00:00');
INSERT INTO `user_log` VALUES (5,3,'log5','2014-01-05 13:00:00');
INSERT INTO `user_log` VALUES (6,3,'log6','2014-01-06 13:00:00');

And I want to query the users joining the latest event logged, I have found two ways to do this but I don't know which one is the best (talking about speed when both tables grow) or if there is another way, here are my queries:

-- Query 1
SELECT
    u.id,
    u.name,
    l2.event as last_record
FROM
    users AS u
INNER JOIN
    (
        SELECT
            idrel,
            MAX(id) as last_id
        FROM user_log
        GROUP BY
            idrel
    ) AS l1
    ON (l1.idrel = u.id)
INNER JOIN
        user_log AS l2
        ON (l2.id = l1.last_id);

Which gives me this result:

+----+--------+-------------+
| id | name   | last_record |
+----+--------+-------------+
|  1 | user_a | log2        |
|  2 | user_b | log4        |
|  3 | user_c | log6        |
+----+--------+-------------+
3 rows in set (0.00 sec)

-- Query 2
SELECT
    u.id,
    u.name,
    (
        SELECT event FROM user_log WHERE idrel = u.id ORDER BY log DESC LIMIT 1
    ) AS last_record
FROM
    users AS u;

And the result:

+----+--------+-------------+
| id | name   | last_record |
+----+--------+-------------+
|  1 | user_a | log2        |
|  2 | user_b | log4        |
|  3 | user_c | log6        |
+----+--------+-------------+
3 rows in set (0.00 sec)

Which one could be the best way? Is there any other better way?

Answer 1

Don’t do it that way, here is a better way, first, add a field to store on the users table the last ID of any new log entry, then create a trigger that updates that field on each new field inserted, then (optional) create a view to have a complete “table” to query from.

-- Add new field to save the modified id
ALTER TABLE `users` ADD `last_mod_id` INT(11) NOT NULL, ADD INDEX (`last_mod_id`);

-- Drop the trigger just in case it exists
DROP TRIGGER IF EXISTS `update_last_id`;

-- Create a trigger to save the last modified id each time a row is created
DELIMITER ;;
CREATE TRIGGER update_last_id 
    AFTER INSERT ON user_log
    FOR EACH ROW
    BEGIN
        UPDATE `users` 
        SET `last_mod_id` = NEW.id
        WHERE `id` = NEW.idrel;
    END;
;;

-- Emtpy your table
TRUNCATE `user_log`;

-- Create all new registries 
INSERT INTO `user_log` VALUES (1,1,'log1','2014-01-01 13:00:00');
INSERT INTO `user_log` VALUES (2,1,'log2','2014-01-02 13:00:00');
INSERT INTO `user_log` VALUES (3,2,'log3','2014-01-03 13:00:00');
INSERT INTO `user_log` VALUES (4,2,'log4','2014-01-04 13:00:00');
INSERT INTO `user_log` VALUES (5,3,'log5','2014-01-05 13:00:00');
INSERT INTO `user_log` VALUES (6,3,'log6','2014-01-06 13:00:00');


-- Query
SELECT
    u.id,
    u.name,
    u.last_mod_id,
    l.event,
    l.log
FROM
    `users` as u
INNER JOIN
    `user_log` as l
    ON (l.id = u.last_mod_id)

-- Drop view if exists
DROP VIEW IF EXISTS `users_log_view`;

-- View
CREATE VIEW `users_log_view` AS 
    SELECT
        u.id,
        u.name,
        u.last_mod_id,
        l.event,
        l.log
    FROM
        `users` as u
    INNER JOIN
        `user_log` as l
        ON (l.id = u.last_mod_id);

-- Query
SELECT * FROM `users_log_view` WHERE `id` = 1;