num int,

valid_from datetime,

valid_until datetime

);

INSERT INTO intervals

(num, valid_from, valid_until)

(1, NOW(), ADDDATE(NOW(), 7)),

(2, ADDDATE(NOW(), 1), ADDDATE(NOW(), 8)),

(3, ADDDATE(NOW(), 2), ADDDATE(NOW(), 9));

FROM (

SELECT ordered_intervals.dt as dt,

SUM(ordered_intervals.dt_ord) OVER (ORDER BY ordered_intervals.dt ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING) AS count

FROM (

SELECT pt.valid_from as dt, 1 as dt_ord FROM intervals pt

UNION

SELECT pt2.valid_until as dt, -1 as dt_ord FROM intervals pt2

ORDER BY dt

) as ordered_intervals

) as sums

WHERE sums.count < 3

ORDER BY sums.dt ASC