CREATE TABLE `email` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `address` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;

INSERT INTO `email` (`address`)
VALUES
    ('alice@example.com'),
    ('bob@example.net'),
    ('carol@example.org'),
    ('dave@example.net'),
    ('eve@example.com'),
    ('frank@example.net');

SELECT COUNT(DISTINCT SUBSTRING_INDEX(`address`, '@', -1)) AS `Unique Domains`
FROM `email`;