MySQL 8.0

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CREATE TABLE heights (
  `id` INTEGER,
  `user_id` INTEGER,
  `created_on` DATETIME,
  `height` INTEGER
);
INSERT INTO heights
  (`id`, `user_id`, `created_on`, `height`)
VALUES
  ('1', '2', '2019-01-01 01:01:00', '180'),
  ('2', '2', '2019-01-02 00:45:00', '181'),
  ('3', '3', '2019-01-03 00:23:00', '182'),
  ('4', '3', '2019-01-04 00:12:00', '183'),
  ('5', '2', '2019-01-07 06:18:00', '184');
CREATE TABLE weights (
  `id` INTEGER,
  `user_id` INTEGER,
  `created_on` DATETIME,
  `weight` INTEGER
);
INSERT INTO weights
  (`id`, `user_id`, `created_on`, `weight`)
VALUES
  ('1', '2', '2019-01-01 00:25:00', '80'),
  ('2', '2', '2019-01-04 04:54:00', '81'),
  ('3', '3', '2019-01-05 00:11:00', '82'),
  ('4', '3', '2019-01-06 00:33:00', '83'),
  ('5', '2', '2019-01-07 05:19:00', '84');
SELECT DISTINCT COALESCE(h.user_id, w.user_id) AS user_id,
       DATE_FORMAT(COALESCE(h.created_on, w.created_on), '%y-%m-%d %H:00:00') AS created_on,
       w.weight,
       h.height
FROM (SELECT created_on FROM heights
      UNION
      SELECT created_on FROM weights) d
LEFT JOIN heights h ON ABS(TIMESTAMPDIFF(HOUR, h.created_on, d.created_on)) = 0 AND h.user_id = 2
LEFT JOIN weights w ON ABS(TIMESTAMPDIFF(HOUR, w.created_on, d.created_on)) = 0 AND w.user_id = 2
WHERE h.user_id IS NOT NULL OR w.user_id IS NOT NULL
ORDER BY created_on


user_id
created_on
weight
height


2
19-01-01 01:00:00
80
180

2
19-01-02 00:00:00
null
181

2
19-01-04 04:00:00
81
null

2
19-01-07 06:00:00
84
184

user_id	created_on	weight	height
2	19-01-01 01:00:00	80	180
2	19-01-02 00:00:00	null	181
2	19-01-04 04:00:00	81	null
2	19-01-07 06:00:00	84	184