CREATE TABLE Rates
(
  fromtime SMALLDATETIME NOT NULL,
  totime   SMALLDATETIME NOT NULL,
  rate     MONEY         NOT NULL,
  PRIMARY KEY(fromtime, totime),
  CHECK (totime > fromtime)
)

INSERT INTO Rates VALUES('00:00', '07:59', 0.02)
INSERT INTO Rates VALUES('08:00', '21:59', 0.05)
INSERT INTO Rates VALUES('22:00', '23:59', 0.02)