SQL合并时间段的问题

假设表结构如下所示：

Tsheets
字段名	字段类型	约束
id	CHAR(10)	PRIMARY KEY
start_date	DATE	CHECK(start_date<= end_date)
end_date	DATE

输入为：
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'
输出为：
start_date      end_date
1997-01-01    1997-01-05
1997-01-06    1997-01-09
1997-01-12    1997-01-15
1997-01-17    1997-01-17

SELECT X.start_date, MIN(X.end_date) as end_date
  FROM (SELECT T1.start_date,T2.end_date
        FROM Tsheets AS T1,Tsheets AS T2,Tsheets AS T3 WHERE T1.end_date <= T2.end_date GROUP BY T1.start_date,T2.end_date
        HAVING MAX (CASE
          WHEN (T1.start_date > T3.start_date
            AND T1.start_date <= T3.end_date)
            OR(T2.end_date >= T3.start_date
            AND T2.end_date < T3.end_date)
            THEN 1 ELSE 0 END) = 0) AS X
   GROUP BY X.start_date