mysql join update

CREATE TABLE IF NOT EXISTS `달력` (
  `idx` int(3) unsigned AUTO_INCREMENT NOT NULL,
  `date` varchar(8)  NOT NULL,  
  PRIMARY KEY (`idx`)
) DEFAULT CHARSET=utf8;
INSERT INTO `달력` (`date`) VALUES
  ('20180101'),
  ('20180102'),
  ('20180103');
  
CREATE TABLE IF NOT EXISTS `생일` (
  `date` varchar(8)  NOT NULL,
  `name` varchar(10) NOT NULL,
  `idx` int(3) unsigned NULL,  
  PRIMARY KEY (`date`, `name`)
) DEFAULT CHARSET=utf8;
INSERT INTO `생일` (`date`, `name`) VALUES
  ('20180101', '흑고니'),
  ('20180102', '백고니');

SELECT * FROM `생일`;

date	name	idx
20180101	흑고니	(null)
20180102	백고니	(null)

UPDATE `생일` a INNER JOIN `달력` b ON a.date = b.date
SET a.idx = b.idx
WHERE a.name='백고니';

SELECT * FROM `생일`;

date	name	idx
20180101	흑고니	(null)
20180102	백고니	2

Test it ! http://sqlfiddle.com/#!9/a90dd0