假设您有一个扩展汇率表，其中包含：
Start Date End Date Rate
========== ========== =======
0001-01-01 2009-01-31 40.1
2009-02-01 2009-02-28 40.1
2009-03-01 2009-03-31 41.0
2009-04-01 2009-04-30 38.5
2009-05-01 9999-12-31 42.7
我们可以讨论前两行是否应该合并的细节，但总体思路是找到给定日期的汇率是微不足道的。此结构与包含范围末端的 SQL 'BETWEEN' 运算符一起使用。通常，更好的范围格式是“开闭”；列出的第一个日期包括在内，第二个日期不包括在内。请注意，数据行有一个限制 - (a) 日期范围的覆盖范围没有间隙，(b) 覆盖范围没有重叠。执行这些约束并非完全微不足道（礼貌的轻描淡写 - 减数分裂）。
现在基本查询是微不足道的，案例 B 不再是特例：
SELECT T.Date, T.Amount, X.Rate
FROM Transactions AS T JOIN ExtendedExchangeRates AS X
ON T.Date BETWEEN X.StartDate AND X.EndDate;
棘手的部分是从给定的 ExchangeRate 表动态创建 ExtendedExchangeRate 表。如果这是一个选项，那么修改基本 ExchangeRate 表的结构以匹配 ExtendedExchangeRate 表将是一个好主意；您在输入数据时（每月一次）而不是每次需要确定汇率时（每天多次）解决混乱的问题。
如何创建扩展汇率表？如果您的系统支持从日期值中加或减 1 以获得下一天或前一天（并且有一个名为“Dual”的单行表），那么对此的变体将起作用（不使用任何 OLAP 函数）：
CREATE TABLE ExchangeRate
(
Date DATE NOT NULL,
Rate DECIMAL(10,5) NOT NULL
);
INSERT INTO ExchangeRate VALUES('2009-02-01', 40.1);
INSERT INTO ExchangeRate VALUES('2009-03-01', 41.0);
INSERT INTO ExchangeRate VALUES('2009-04-01', 38.5);
INSERT INTO ExchangeRate VALUES('2009-05-01', 42.7);
第一排：
SELECT '0001-01-01' AS StartDate,
(SELECT MIN(Date) - 1 FROM ExchangeRate) AS EndDate,
(SELECT Rate FROM ExchangeRate
WHERE Date = (SELECT MIN(Date) FROM ExchangeRate)) AS Rate
FROM Dual;
结果：
0001-01-01 2009-01-31 40.10000
最后一行：
SELECT (SELECT MAX(Date) FROM ExchangeRate) AS StartDate,
'9999-12-31' AS EndDate,
(SELECT Rate FROM ExchangeRate
WHERE Date = (SELECT MAX(Date) FROM ExchangeRate)) AS Rate
FROM Dual;
结果：
2009-05-01 9999-12-31 42.70000
中间行：
SELECT X1.Date AS StartDate,
X2.Date - 1 AS EndDate,
X1.Rate AS Rate
FROM ExchangeRate AS X1 JOIN ExchangeRate AS X2
ON X1.Date < X2.Date
WHERE NOT EXISTS
(SELECT *
FROM ExchangeRate AS X3
WHERE X3.Date > X1.Date AND X3.Date < X2.Date
);
结果：
2009-02-01 2009-02-28 40.10000
2009-03-01 2009-03-31 41.00000
2009-04-01 2009-04-30 38.50000
请注意，NOT EXISTS 子查询相当重要。没有它，“中间行”的结果是：
2009-02-01 2009-02-28 40.10000
2009-02-01 2009-03-31 40.10000 # Unwanted
2009-02-01 2009-04-30 40.10000 # Unwanted
2009-03-01 2009-03-31 41.00000
2009-03-01 2009-04-30 41.00000 # Unwanted
2009-04-01 2009-04-30 38.50000
随着表大小的增加，不需要的行数急剧增加（对于 N > 2 行，我相信有 (N-2) * (N - 3) / 2 个不需要的行）。
ExtendedExchangeRate 的结果是三个查询的（不相交的）UNION：
SELECT DATE '0001-01-01' AS StartDate,
(SELECT MIN(Date) - 1 FROM ExchangeRate) AS EndDate,
(SELECT Rate FROM ExchangeRate
WHERE Date = (SELECT MIN(Date) FROM ExchangeRate)) AS Rate
FROM Dual
UNION
SELECT X1.Date AS StartDate,
X2.Date - 1 AS EndDate,
X1.Rate AS Rate
FROM ExchangeRate AS X1 JOIN ExchangeRate AS X2
ON X1.Date < X2.Date
WHERE NOT EXISTS
(SELECT *
FROM ExchangeRate AS X3
WHERE X3.Date > X1.Date AND X3.Date < X2.Date
)
UNION
SELECT (SELECT MAX(Date) FROM ExchangeRate) AS StartDate,
DATE '9999-12-31' AS EndDate,
(SELECT Rate FROM ExchangeRate
WHERE Date = (SELECT MAX(Date) FROM ExchangeRate)) AS Rate
FROM Dual;
在测试 DBMS（MacOS X 10.6.2 上的 IBM Informix Dynamic Server 11.50.FC6）上，我能够将查询转换为视图，但我不得不停止使用数据类型作弊 - 通过将字符串强制转换为日期：
CREATE VIEW ExtendedExchangeRate(StartDate, EndDate, Rate) AS
SELECT DATE('0001-01-01') AS StartDate,
(SELECT MIN(Date) - 1 FROM ExchangeRate) AS EndDate,
(SELECT Rate FROM ExchangeRate WHERE Date = (SELECT MIN(Date) FROM ExchangeRate)) AS Rate
FROM Dual
UNION
SELECT X1.Date AS StartDate,
X2.Date - 1 AS EndDate,
X1.Rate AS Rate
FROM ExchangeRate AS X1 JOIN ExchangeRate AS X2
ON X1.Date < X2.Date
WHERE NOT EXISTS
(SELECT *
FROM ExchangeRate AS X3
WHERE X3.Date > X1.Date AND X3.Date < X2.Date
)
UNION
SELECT (SELECT MAX(Date) FROM ExchangeRate) AS StartDate,
DATE('9999-12-31') AS EndDate,
(SELECT Rate FROM ExchangeRate WHERE Date = (SELECT MAX(Date) FROM ExchangeRate)) AS Rate
FROM Dual;