合并时间段：
将连续或者重叠的时间段合并。
create table timesheets (
task_id char(10) not null primary key,
startdate date not null,
enddate date not null,
check(startdate<=enddate))
insert into timesheets values
(1,'1997-01-01','1997-01-03'),
(2,'1997-01-02','1997-01-04'),
(3,'1997-01-04','1997-01-05'),
(4,'1997-01-06','1997-01-09'),
(5,'1997-01-09','1997-01-09'),
(6,'1997-01-09','1997-01-09'),
(7,'1997-01-12','1997-01-15'),
(8,'1997-01-13','1997-01-14'),
(9,'1997-01-14','1997-01-14'),
(10,'1997-01-17','1997-01-17')
select t1.startdate,max(t2.enddate) from timesheets as t1,timesheets t2 where t1.startdate<=t2.enddate
and not exists (
select * from timesheets as t3,timesheets t4 where
t3.enddate<t4.startdate and
t3.startdate>=t1.startdate and
t3.enddate<=t2.enddate and
t4.startdate>=t1.startdate and
t4.enddate<=t2.enddate
and not exists (
select * from timesheets t5 where t5.startdate between t3.startdate and t3.enddate
and t5.enddate between t4.startdate and t4.enddate
)
)
group by t1.startdate having t1.startdate=min(t2.startdate)