Не знаю, оно ли, но у меня первая идея была сделать это так:
SELECT
date(event_time) as event_date
, extract(HOUR_MINUTE from event_time) as hour_min
, extract(SECOND from event_time) as sec
, count(*) as cnt
FROM table1
where event_time is not null
group by date(event_time), extract(HOUR_MINUTE from event_time), extract(SECOND from event_time);
Затем попроще:
SELECT
`event_time` - INTERVAL EXTRACT(SECOND FROM `event_time`) SECOND as no_sec
, count(*) as cnt
FROM table1
group by `event_time` - INTERVAL EXTRACT(SECOND FROM `event_time`) SECOND
order by event_time;