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：
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));
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');
解惑 #1
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);
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
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
解惑 #2
这个查询很长，但是检查一下查询时间。
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(start_date)
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(end_date)
ON X.start_date <= Y.end_date
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
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
解惑 #3
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
这个小小的查询中包含了很多逻辑。