MySQL 8.0

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CREATE TABLE mytable(
   Id       INT
  ,userName VARCHAR(10)
  ,Browser  VARCHAR(9)
  ,Platform VARCHAR(10)
  ,TS       timestamp
);
INSERT INTO mytable(Id,userName,Browser,Platform,TS) VALUES (1,'abc','Firefox',NULL,'2006-10-05 11:55:45');
INSERT INTO mytable(Id,userName,Browser,Platform,TS) VALUES (2,'xyz','Chrome',NULL,'2007-10-07 12:34:17');
INSERT INTO mytable(Id,userName,Browser,Platform,TS) VALUES (3,'mnp','Safari',NULL,'2008-10-09 08:19:37');
INSERT INTO mytable(Id,userName,Browser,Platform,TS) VALUES 
  (4,'abc','Safari',NULL,'2010-10-13 04:28:14')
, (5,'abc',NULL,'Windows','2006-01-01 12:02:45')
, (6,'xyz',NULL,'Linux','2007-01-01 12:01:20')
, (7,'mnp',NULL,'MAC','2008-01-01 12:02:29')
, (8,'abc',NULL,'MAC','2010-03-09 13:06:59')
with cte as (
select
*
, coalesce(lead(ts) over(order by ts),current_time) nxt
from mytable
)
select
  id
, username
, coalesce(browser,(select browser from cte where t.ts between cte.ts and cte.nxt limit 1 )) browser
, coalesce(platform,(select platform from cte where t.ts between cte.ts and cte.nxt limit 1 )) platform
, ts
, nxt
from cte t



id
username
browser
platform
ts
nxt


5
abc
null
Windows
2006-01-01 12:02:45
2006-10-05 11:55:45

1
abc
Firefox
Windows
2006-10-05 11:55:45
2007-01-01 12:01:20

6
xyz
Firefox
Linux
2007-01-01 12:01:20
2007-10-07 12:34:17

2
xyz
Chrome
Linux
2007-10-07 12:34:17
2008-01-01 12:02:29

7
mnp
Chrome
MAC
2008-01-01 12:02:29
2008-10-09 08:19:37

3
mnp
Safari
MAC
2008-10-09 08:19:37
2010-03-09 13:06:59

8
abc
Safari
MAC
2010-03-09 13:06:59
2010-10-13 04:28:14

4
abc
Safari
MAC
2010-10-13 04:28:14
2021-02-10 08:33:52

id	username	browser	platform	ts	nxt
5	abc	null	Windows	2006-01-01 12:02:45	2006-10-05 11:55:45
1	abc	Firefox	Windows	2006-10-05 11:55:45	2007-01-01 12:01:20
6	xyz	Firefox	Linux	2007-01-01 12:01:20	2007-10-07 12:34:17
2	xyz	Chrome	Linux	2007-10-07 12:34:17	2008-01-01 12:02:29
7	mnp	Chrome	MAC	2008-01-01 12:02:29	2008-10-09 08:19:37
3	mnp	Safari	MAC	2008-10-09 08:19:37	2010-03-09 13:06:59
8	abc	Safari	MAC	2010-03-09 13:06:59	2010-10-13 04:28:14
4	abc	Safari	MAC	2010-10-13 04:28:14	2021-02-10 08:33:52