MySQL: Find "last modified" date from two tables - mysql

Question

I have two MySQL tables: tech_requests and comments. I want to display each tech_request one time in a list ordered by the "last modified" date, whether that be the date of the tech_request creation or the latest comment tied to that tech_request. I was trying to use UNION but I got stuck. Any ideas would be much appreciated. Here are the tables:
CREATE TABLE `tech_requests` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`staff_member_id` int(3) NOT NULL,
`date_time` datetime NOT NULL,
`request` text NOT NULL,
`building_id` int(2) NOT NULL,
`technician_id` int(2) DEFAULT NULL,
`completed` tinyint(1) NOT NULL,
`subject` varchar(30) NOT NULL DEFAULT '',
`category_id` int(2) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=203 DEFAULT CHARSET=utf8;
CREATE TABLE `comments` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`tech_request_id` int(11) NOT NULL,
`technician_id` int(2) NOT NULL,
`date_time` datetime NOT NULL,
`comment` text NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=234 DEFAULT CHARSET=utf8;

Answer 1

Are you looking for something like this?
SELECT r.id, r.staff_member_id, ...,
GREATEST(r.date_time, COALESCE(c.date_time, 0)) last_modified
FROM tech_requests r LEFT JOIN
(
SELECT tech_request_id, MAX(date_time) date_time
FROM comments c
GROUP BY tech_request_id
) c
ON r.id = c.tech_request_id
ORDER BY last_modified
Here is SQLFiddle demo