假設 (id,StartDate,EndDate) 是「唯一」:
select id,sum(day) day
from (
select id,StartDate,EndDate,min(day) day
from (
select t2.id,t2.StartDate,t2.EndDate,
case
when t1.StartDate = t2.StartDate and t1.EndDate = t2.EndDate
then t2.day