Akina

SELECT brand.b_id
FROM brand
NATURAL LEFT JOIN model 
NATURAL LEFT JOIN item
GROUP BY 1
HAVING NOT SUM(i_id IS NULL)

CREATE TABLE users (
  id INT AUTO_INCREMENT PRIMARY KEY,
  username VARCHAR(100),
  created_at DATETIME DEFAULT CURRENT_TIMESTAMP
);

CREATE TRIGGER tr_users_bi
BEFORE INSERT ON users
FOR EACH ROW
BEGIN
  DECLARE allowed_created_at DATETIME;
  DECLARE error_message_text VARCHAR(64);
  SELECT MAX(created_at) + INTERVAL 12 HOUR INTO allowed_created_at
  FROM users
  WHERE username = NEW.username;
  IF NEW.created_at < allowed_created_at THEN
    SET error_message_text = CONCAT('Регистрация возможна не ранее ', allowed_created_at);
    SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = error_message_text; 
  END IF;
END

Количество(Склад Номер Х) = ВсегоТовара * НомерСклада / ВсегоСкладов - Количество(Склады Номер 1 .. Х-1)

Склад 1: количество = 5 * 1 / 3 - 0 = 1,666 = 2 штуки
Склад 2: количество = 5 * 2 / 3 - 2 = 1,333 = 1 штука
Склад 3: количество = 5 * 3 / 3 - (2 + 1) = 1,666 = 2 штуки

$amount = 5;
$num = 3;

for($used = 0, $i = 1; $i <= $num; $i++) {
  $used += ($current = round($amount * $i / $num - $used));
  echo("$i: $current\n");
}

SELECT c.name category_name, p.id post_id, p.name post_name, p.title post_title
FROM posts p
JOIN categories c ON p.category_id = c.id
UNION ALL
SELECT name, NULL, NULL, NULL
FROM categories 
ORDER BY category_name, post_id IS NOT NULL

Или можно как-то в PDO подключаться не сразу к бд