CREATE TABLE `currencies` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `community_id` int(10) unsigned NOT NULL,
  `currency` varchar(3) NOT NULL,
  `round_digits` tinyint(1) unsigned NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `unique_currency_for_community` (`community_id`, `currency`),
  KEY `community_id` (`community_id`),
  KEY `currency` (`currency`),
  CONSTRAINT `currencies_community_id` FOREIGN KEY (`community_id`) REFERENCES `communities` (`id`)
) ENGINE = InnoDB DEFAULT CHARSET = utf8mb4;

CREATE TABLE `currency_exchange_rates` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `currency_id` int(10) unsigned NOT NULL,
  `exchange_rate` decimal(19,9) unsigned NOT NULL,
  `valid_from` timestamp NOT NULL DEFAULT current_timestamp(),
  PRIMARY KEY (`id`),
  KEY `currency_id` (`currency_id`),
  INDEX `valid_from` (`valid_from`),
  CONSTRAINT `currency_exchange_rates_currency_id` FOREIGN KEY (`currency_id`) REFERENCES `currencies` (`id`)
) ENGINE = InnoDB DEFAULT CHARSET = utf8mb4;