MariaDB 10.2

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
CREATE TABLE `archive_day_ET` (
  `dateTime` int(11) NOT NULL,
  `min` double DEFAULT NULL,
  `mintime` int(11) DEFAULT NULL,
  `max` double DEFAULT NULL,
  `maxtime` int(11) DEFAULT NULL,
  `sum` double DEFAULT NULL,
  `count` int(11) DEFAULT NULL,
  `wsum` double DEFAULT NULL,
  `sumtime` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
CREATE TABLE `archive_day_inTemp` (
  `dateTime` int(11) NOT NULL,
  `min` double DEFAULT NULL,
  `mintime` int(11) DEFAULT NULL,
  `max` double DEFAULT NULL,
  `maxtime` int(11) DEFAULT NULL,
  `sum` double DEFAULT NULL,
  `count` int(11) DEFAULT NULL,
  `wsum` double DEFAULT NULL,
  `sumtime` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
SELECT CONCAT('FROM (\r\n ',GROUP_CONCAT(CONCAT('SELECT `dateTime` FROM ',TABLE_NAME) SEPARATOR '\r\n UNION\r\n '),'\r\n) AS dt \r\nLEFT JOIN ',
 GROUP_CONCAT(CONCAT(TABLE_NAME,' ON ',
 TABLE_NAME,'.`dateTime` = dt.`dateTime`') SEPARATOR '\r\nLEFT JOIN ')) as SqlJoins
FROM information_schema.tables
WHERE TABLE_NAME like 'archive_day_%'


SqlJoins


FROM (
 SELECT `dateTime` FROM archive_day_ET
 UNION
 SELECT `dateTime` FROM archive_day_inTemp
) AS dt 
LEFT JOIN archive_day_ET ON archive_day_ET.`dateTime` = dt.`dateTime`
LEFT JOIN archive_day_inTemp ON archive_day_inTemp.`dateTime` = dt.`dateTime`