连接上不同表的平均值

CREATE TABLE `reviews` (
  `id` int(11) NOT NULL,
  `average` decimal(11,2) NOT NULL,
  `house_id` int(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
INSERT INTO `reviews` (`id`, `average`, `house_id`) VALUES
(1, '10.00', 1),
(2, '10.00', 1);
ALTER TABLE `reviews`
  ADD PRIMARY KEY (`id`);
ALTER TABLE `reviews`
  MODIFY `id` int(11) NOT NULL AUTO_INCREMENT, AUTO_INCREMENT=3;

CREATE TABLE `dummy_reviews` (
  `id` int(11) NOT NULL,
  `average` decimal(11,2) NOT NULL,
  `house_id` int(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
INSERT INTO `dummy_reviews` (`id`, `average`, `house_id`) VALUES
(0, '2.00', 1);
ALTER TABLE `dummy_reviews`
  ADD PRIMARY KEY (`id`);
CREATE TABLE `reviews` (
  `id` int(11) NOT NUL