SELECT * FROM `table` WHERE `field`="123" LIMIT 1
$descr = query( SELECT `description` FROM `menu_items` WHERE `id` = $id LIMIT 1 )
SELECT
`c`.`id` as `id`,
`c`.`name` as `name`,
`i`.`id` as `item_id`,
`i`.`name` as `item_name`,
`i`.`url` as `item_url`
`i`.`description` as `item_descr`
FROM `menu_category` as `c`
LEFT JOIN `menu_items` as `i` ON `i`.`menu_category_id`=`c`.`id`
ORDER BY `c`.`order`, `i`.`order`
echo '<li><a href="#" onclick="alert("'.$item['descr'].'")">'.$item['name'].'</a></li>';