MariaDB 10.2

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create table switching_log
(
    item_name varchar(20) NOT NULL,
    switched_off timestamp NULL DEFAULT NULL,
    switched_on timestamp NULL DEFAULT NULL
);
insert into switching_log(item_name) values
('no explicit date');
insert into switching_log values
('null, null', null, null),
('date, null', '2019-10-01 08:40:28', null),
('null, date', null, '2019-10-01 08:35:49'),
('date, date', '2019-10-01 08:40:28', '2019-10-01 08:35:49');
select * from switching_log


item_name
switched_off
switched_on


no explicit date
null
null

null, null
null
null

date, null
2019-10-01 08:40:28
null

null, date
null
2019-10-01 08:35:49

date, date
2019-10-01 08:40:28
2019-10-01 08:35:49


update switching_log set switched_on = current_timestamp;
select * from switching_log


item_name
switched_off
switched_on


no explicit date
null
2019-10-01 10:04:12

null, null
null
2019-10-01 10:04:12

date, null
2019-10-01 08:40:28
2019-10-01 10:04:12

null, date
null
2019-10-01 10:04:12

date, date
2019-10-01 08:40:28
2019-10-01 10:04:12

item_name	switched_off	switched_on
no explicit date	null	null
null, null	null	null
date, null	2019-10-01 08:40:28	null
null, date	null	2019-10-01 08:35:49
date, date	2019-10-01 08:40:28	2019-10-01 08:35:49

item_name	switched_off	switched_on
no explicit date	null	2019-10-01 10:04:12
null, null	null	2019-10-01 10:04:12
date, null	2019-10-01 08:40:28	2019-10-01 10:04:12
null, date	null	2019-10-01 10:04:12
date, date	2019-10-01 08:40:28	2019-10-01 10:04:12