MySQL 8.0

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 CREATE TABLE IF NOT EXISTS `sales` (
  `id` int(2) NOT NULL,
  `date` varchar(50),
  `price` int(6),
  PRIMARY KEY (`id`)
) DEFAULT CHARSET=utf8;
INSERT INTO `sales` (`id`, `date`, `price`) VALUES
  ('1', '2019-01-06','25000');

 CREATE TABLE IF NOT EXISTS `purchase` (
  `id` int(2) NOT NULL,
  `date` varchar(50),
  `price` int(6),
  PRIMARY KEY (`id`)
) DEFAULT CHARSET=utf8;
SELECT GROUP_CONCAT(CONCAT_WS(',', sales.date, purchase.date)) AS date,
       GROUP_CONCAT(COALESCE(sales.date, ''), ',', coalesce(purchase.date, '')),
       GROUP_CONCAT(COALESCE(purchase.date, ''), ',', coalesce(sales.date, ''))
FROM sales LEFT JOIN purchase ON 1=1;


date
GROUP_CONCAT(COALESCE(sales.date, ''), ',', coalesce(purchase.date, ''))
GROUP_CONCAT(COALESCE(purchase.date, ''), ',', coalesce(sales.date, ''))


2019-01-06
2019-01-06,
,2019-01-06