Более одного раза для одного столбца:
SELECT count(pn1)
FROM pledges
WHERE date1 BETWEEN '2011-05-05' AND '2011-06-06'
OR date1 BETWEEN '2011-07-05' AND '2011-08-06'
Два разных столбца:
SELECT count(pn1)
FROM pledges
WHERE date1 BETWEEN '2011-05-05' AND '2011-06-06'
AND date2 BETWEEN '2011-05-05' AND '2011-06-06'
РЕДАКТИРОВАТЬ : Исходя из комментариев, возможно, вы ищете что-то подобное вместо этого?
SELECT SUM(CASE WHEN date1 BETWEEN '2011-05-05' AND '2011-06-06' THEN 1 ELSE 0) END AS Count1,
SUM(CASE WHEN date1 BETWEEN '2011-07-05' AND '2011-08-06' THEN 1 ELSE 0) END AS Count2
FROM pledges
WHERE date1 BETWEEN '2011-05-05' AND '2011-06-06'
OR date1 BETWEEN '2011-07-05' AND '2011-08-06'