Rsa97

SELECT *
  FROM `table`
  WHERE `field` = :value
UNION (
  SELECT *
    FROM `table`
    WHERE `field` > :value
    ORDER BY `field` DESC
    LIMIT 1)
UNION (
  SELECT *
    FROM `table`
    WHERE `field` < :value
    ORDER BY `field`
    LIMIT 1)

$$
DELIMITER ;

для всех i от n  до 2 выполнить
  j ← случайное число 1 ≤ j ≤ i
  обменять местами a[j] и a[i]

SELECT `t1`.`think_id`, `t1`.`user_id`, `t2`.`user_id`
  FROM `table` AS `t1`
  JOIN `table` AS `t2` ON `t1`.`user_id` = :user1 AND `t2`.`user_id` = :user2
    AND `t2`.`think_id` = `t1`.`think_id`

SELECT `q`.`id_cat`, `a`.`id_user`, SUM(`a`.`res`) AS `res`
  FROM `users` AS `u`
  JOIN `questions` AS `q` 
  LEFT JOIN `answers` AS `a` ON `a`.`id_vopr` = `q`.`id` AND `a`.`id_user` = `u`.`id`
  GROUP BY `q`.`id_cat`, `a`.`user`
  HAVING COUNT(*) = COUNT(`a`.`id_vopr`)