MySQL 8.0

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create table test (id int, t timestamp(3) default current_timestamp(3))
insert into test (id) values (4);
select * from test

id
t

4
2019-04-01 08:51:08.270

select unix_timestamp(t) from test

unix_timestamp(t)

1554105068.270

insert into test values (6, from_unixtime(1554102345.353))
select * from test

id
t

4
2019-04-01 08:51:08.270

6
2019-04-01 08:05:45.353