SELECT a.timePeriod as t, b.count as c from (
with (select toUInt32(dateDiff('day', [START_DATE], [END_DATE])) )
as diffInTimeUnits
select arrayJoin(arrayMap(x -> (toDate(addDays([START_DATE], x))), range(0, diffInTimeUnits+1))) as timePeriod ) a
LEFT JOIN
(select count(*) as count, toDate(toStartOfDay(started_at)) as timePeriod from logs WHERE
[CONDITIONS]
GROUP BY toStartOfDay(started_at)) b on a.timePeriod=b.timePeriod