Optimizing sql statements to reduce MySQL server load - php

Question

$sql1 = "SELECT questions FROM last_check_date WHERE user_id=? ORDER BY questions DESC LIMIT 1";
$sql2 = "SELECT id FROM questions WHERE add_dt>?";
What do statements above do?
When I execute sql1, it gets last check date for user.
Then I'm executing second query, to fetch all id's where add date>last check date (from sql1) and return affected rows count.
What I want to do is to merge this 2 statements into 1, and optimize query count. Following problem may occur:
There is no row for user in $sql1: must select all rows in sql2 and return affected rows count.
I can't figure out how it must look like.. Thx in advance
UPDATE
SHOW CREATE TABLE last_check_date; result is
CREATE TABLE `last_check_date` (
`id` int(11) unsigned NOT NULL,
`user_id` bigint(20) unsigned NOT NULL,
`questions` datetime DEFAULT NULL,
`users` datetime DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
And SHOW CREATE TABLE questions;
CREATE TABLE `questions` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`author_id` bigint(20) unsigned DEFAULT NULL,
`question` text NOT NULL,
`var_a` text NOT NULL,
`var_b` text NOT NULL,
`var_c` text NOT NULL,
`var_d` text NOT NULL,
`var_e` text NOT NULL,
`subject` int(11) unsigned DEFAULT NULL,
`chapter` int(11) unsigned DEFAULT NULL,
`section` int(11) unsigned DEFAULT NULL,
`paragraph` int(11) unsigned DEFAULT NULL,
`rank` tinyint(2) NOT NULL,
`add_dt` datetime NOT NULL,
`answer` varchar(1) NOT NULL,
PRIMARY KEY (`id`),
KEY `fk_chapters-id` (`chapter`),
KEY `fk_paragraphs-id` (`paragraph`),
KEY `fk_subjects-id` (`subject`),
KEY `fk_sections-id` (`section`),
KEY `fk_author-id` (`author_id`),
CONSTRAINT `fk_author-id` FOREIGN KEY (`author_id`) REFERENCES `users` (`id`) ON DELETE SET NULL ON UPDATE CASCADE,
CONSTRAINT `fk_chapters-id` FOREIGN KEY (`chapter`) REFERENCES `chapters` (`id`) ON DELETE SET NULL ON UPDATE CASCADE,
CONSTRAINT `fk_paragraphs-id` FOREIGN KEY (`paragraph`) REFERENCES `paragraphs` (`id`) ON DELETE SET NULL ON UPDATE CASCADE,
CONSTRAINT `fk_sections-id` FOREIGN KEY (`section`) REFERENCES `sections` (`id`) ON DELETE SET NULL ON UPDATE CASCADE,
CONSTRAINT `fk_subjects-id` FOREIGN KEY (`subject`) REFERENCES `subjects` (`id`) ON DELETE SET NULL ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8

Answer 1

$sql = "
SELECT q.id
FROM questions q
LEFT JOIN (
SELECT questions
FROM last_check_date
WHERE user_id=?
ORDER BY questions
DESC LIMIT 1
) l ON q.add_dt > l.questions"
$rs = mysql_query($sql);
$rowcount = mysql_num_rows($rs);
I don't know yet the proper syntax for PDO/MYSQLI, please adapt to your prefered driver.

Answer 2

See below. I have assumed that if there are no records in last_check_date that you still want to show questions (in that case all of them).
select q.id
from questions q
left outer join (
select max(questions) as questions
from last_check_date
where user_id = ?
) lcd on q.add_date > lcd.questions
where user_id = ?
order by questions desc