假设有一张考勤单，经常需要将连续或重叠的时间段合并。在做简单查询时这可能成为问题，所以要当心，这个问题并不容易领会或理解：

创建示例表与数据：

CREATE TABLE Timesheets (

task_id CHAR(10) NOT NULL PRIMARY KEY,

start_date DATE NOT NULL,

end_date DATE NOT NULL,

CHECK(start_date <= end_date)

);

NSERT 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');

方案一：(总耗时  : 0.004 sec)

SELECT

T1.start_date,

MAX(T2.end_date)

FROM

Timesheets AS T1,

Timesheets AS T2

WHERE T1.start_date <= T2.end_date

AND NOT EXISTS

(SELECT

*

FROM

Timesheets AS T3,

Timesheets AS T4

WHERE T3.end_date < T4.start_date

AND T3.start_date >= T1.start_date

AND T3.end_date <= T2.end_date

AND T4.start_date >= T1.start_date

AND T4.end_date <= T2.end_date

AND NOT EXISTS

(SELECT

*

FROM

Timesheets AS T5

WHERE T5.start_date BETWEEN T3.start_date

AND T3.end_date

AND T5.end_date BETWEEN T4.start_date

AND T4.end_date))

GROUP BY T1.start_date

HAVING T1.start_date = MIN(t2.start_date) ;

方案二：(总耗时 : 0.002 sec)

SELECT

X.start_date,

MIN(Y.end_date) AS end_date

FROM

(SELECT

T1.start_date

FROM

Timesheets AS T1

LEFT OUTER JOIN Timesheets AS T2

ON T1.start_date > T2.start_date

AND T1.start_date <= T2.end_date

GROUP BY T1.start_date

HAVING COUNT(T2.start_date) = 0) AS X

INNER JOIN

(SELECT

T3.end_date

FROM

Timesheets AS T3

LEFT OUTER JOIN Timesheets AS T4

ON T3.end_date >= T4.start_date

AND T3.end_date < T4.end_date

GROUP BY T3.end_date

HAVING COUNT(T4.start_date) = 0) AS Y

ON X.start_date <= Y.end_date

GROUP BY X.start_date ;

方案三：(总耗时 : 0.002 sec)

SELECT

X.start_date,

MIN(X.end_date) AS end_date

FROM

(SELECT

T1.start_date,

T2.end_date

FROM

Timesheets AS T1,

Timesheets AS T2,

Timesheets 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 ;

结果：

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