当外键中需要更为复杂的逻辑表达式时
考虑下面的简单常识： 您的设备的最大电流不能超过您插入到它的电路的最大电流。 假设下面的表存储电路和设备数据：
代码如下:
CREATE TABLE Data.Curcuits(CurcuitID INT NOT NULL
CONSTRAINT PK_Curcuits PRIMARY KEY,
MaximumCurrent INT NOT NULL,
Description VARCHAR(100) NOT NULL);
GO
INSERT INTO Data.Curcuits(CurcuitID,
MaximumCurrent,
Description)
SELECT 1, 25, 'Deck and Garage';
GO
CREATE TABLE Data.Devices(DeviceID INT NOT NULL
CONSTRAINT PK_Devices PRIMARY KEY,
CurcuitID INT NULL,
MaximumCurrent INT NOT NULL,
Description VARCHAR(100) NOT NULL,
CONSTRAINT FK_Devices_Curcuits FOREIGN KEY(CurcuitID)
REFERENCES Data.Curcuits(CurcuitID)
);
GO
It would be very convenient to issue a simple command and implement this business rule:
一个非常简便的命令就可能实现这个业务规则：
ALTER TABLE Data.Devices ADD CONSTRAINT FK_Devices_Curcuits
FOREIGN KEY(CurcuitID, MaximumCurrent)
REFERENCES Data.Curcuits(CurcuitID, MaximumCurrent)
MATCH ON((Data.Devices.CurcuitID = Data.Curcuits.CurcuitID) AND
(Data.Devices.MaximumCurrent = MaximumCurrent);
GO
You can verify that the constraints work:
你可以验证该约束有效：
INSERT INTO Data.Devices(DeviceID,
CurcuitID,
MaximumCurrent,
CurcuitMaximumCurrent,
Description)
SELECT 1, 1, 50, 25, 'Electric car charger'
Msg 547, Level 16, State 0, Line 1
The INSERT statement conflicted with the CHECK constraint "CHK_Devices_SufficientCurcuitMaximumCurrent". The conflict occurred in database "Test", table "data.Devices".
The statement has been terminated.
INSERT 语句和CHECK约束"CHK_Devices_SufficientCurcuitMaximumCurrent"发生冲突。 该冲突发生在数据库"Test"的"data.Devices"表。
该语句被终止执行。
As you have seen, the implementation of a very simple and very common business rule is quite involved, because such business rules are not directly supported by the database engine.
可以看出，一个非常简单而普通的业务规则实现起来也相当繁杂，因为数据库引擎并不直接支持这种业务规则。
When you want to create constraints on indexed views
在索引视图上创建约束
Even when your database guarantees that “the maximum current of your device cannot exceed the maximum current of the circuit you plug it into”, it is not good enough. Consider the following sample data:
尽管数据库保证“您的设备的最大电流不能超过您插入到它的电路的最大电流”，但这还不够。请看下列示例数据：
INSERT INTO Data.Devices(DeviceID,
CurcuitID,
MaximumCurrent,
CurcuitMaximumCurrent,
Description)
SELECT 2, 1, 15, 25, 'ShopVac';
INSERT INTO Data.Devices(DeviceID,
CurcuitID,
MaximumCurrent,
CurcuitMaximumCurrent,
Description)
SELECT 3, 1, 15, 25, 'Miter Saw';
The database structure allows to plug more than one device into a circuit, which is correct, but if you turn both devices on, their combined maximum current exceeds the circuit's maximum current. To enforce this business rule, it would be natural to create an indexed view, so that the database guarantees that the totals are always correct:
数据库中的数据表明可以插入一个以上的设备到电路，这没有错，可是当所有的设备都打开时，它们的最大电流之和会超过电路最大电流。为了加强这个业务规则，很自然的会创建一个索引视图以使数据库保证电流之和总是正确的。
CREATE VIEW Data.TotalMaximumCurrentPerCircuit WITH SCHEMABINDING
AS
SELECT d.CurcuitID,
c.MaximumCurrent AS CircuitMaximumCurrent,
SUM(d.MaximumCurrent) AS TotalMaximumCurrent,
COUNT_BIG(*) AS NumDevices
FROM Data.Devices d JOIN Data.Curcuits c ON d.CurcuitID = c.CurcuitID
GROUP BY d.CurcuitID, c.MaximumCurrent;
GO
CREATE UNIQUE CLUSTERED INDEX Data_TotalMaximumCurrentPerCircuit
ON Data.TotalMaximumCurrentPerCircuit(CurcuitID);
GO
If I could create a check constraint on that indexed view, I would be all set:
如果能在该索引视图上创建一个约束，我将进行这样的设置：
ALTER VIEW Data.TotalMaximumCurrentPerCircuit
ADD CONSTRAINT CHK_TotalMaximumCurrentPerCircuit_ValidCurcuit
CHECK(TotalMaximumCurrent