确保为一组定义一个且仅有一个默认值

我有一个与地址表具有一对多关系的客户表。 我想限制数据库以便客户with地址将始终有一个（且只有一个）默认地址。

我可以很容易地添加一个约束，以确保每个客户只有一个默认地址。然而，我正在努力解决如何应用一个约束来确保地址始终被标记为默认地址。

总结一下：

• 客户不需要有任何地址。
• 如果客户有地址，则必须有一个默认地址。
• 每个客户只能有一个默认地址。

这是问题的示例和一些“单元”测试。我正在使用链接表来连接客户和地址。

CREATE TABLE Customer
(
    Id INT PRIMARY KEY,
    Name VARCHAR(100) NOT NULL
)

CREATE TABLE [Address]
(
    Id INT PRIMARY KEY,
    Address VARCHAR(500) NOT NULL
)

CREATE TABLE CustAddress
(
    CustomerId INT,
    AddressId INT,
    [Default] BIT NOT NULL,
    FOREIGN KEY (CustomerId) REFERENCES Customer(Id),
    FOREIGN KEY (AddressId) REFERENCES [Address](Id)
)

INSERT INTO Customer VALUES (1, 'Mr Greedy')

INSERT INTO [Address] VALUES (1, 'Roly-Poly House, Fatland')
INSERT INTO [Address] VALUES (2, 'Giant Cottage, A Cave')

-- Should succeed
INSERT INTO CustAddress VALUES (1, 1, 1)
INSERT INTO CustAddress VALUES (1, 2, 0)

DELETE FROM CustAddress

-- Should fail as no default address set
INSERT INTO CustAddress VALUES (1, 1, 0)

DELETE FROM CustAddress

-- Should fail as we end up with no defualt address set
INSERT INTO CustAddress VALUES (1, 1, 1)
INSERT INTO CustAddress VALUES (1, 2, 0)
UPDATE CustAddress SET [Default] = 0 WHERE CustomerId = 1 AND AddressId = 1

DELETE FROM CustAddress

-- Should fail as we end up with no defualt address set
INSERT INTO CustAddress VALUES (1, 1, 1)
INSERT INTO CustAddress VALUES (1, 2, 0)
DELETE FROM CustAddress WHERE CustomerId = 1 AND AddressId = 1

如何将架构更改为

CREATE TABLE Customer
(
    Id INT PRIMARY KEY,
    Name VARCHAR(100) NOT NULL
)

CREATE TABLE [Address]
(
    Id INT PRIMARY KEY,
    Address VARCHAR(500) NOT NULL
)


CREATE TABLE CustDefaultAddress
(
    CustomerId INT PRIMARY KEY, /*Ensures no more than one default*/
    AddressId INT,
    FOREIGN KEY (CustomerId) REFERENCES Customer(Id),
    FOREIGN KEY (AddressId) REFERENCES [Address](Id)
)


CREATE TABLE CustSecondaryAddress
(
    CustomerId INT REFERENCES CustDefaultAddress(CustomerId), 
                   /*No secondary address can be added unless default one exists*/
    AddressId INT REFERENCES [Address](Id),
    PRIMARY KEY(CustomerId, AddressId)
)

如果存在附加要求，即地址不得同时作为主地址和辅助地址出现，您可以使用辅助表和索引视图强制执行此要求。

CREATE TABLE dbo.TwoRows
  (
     X INT PRIMARY KEY
  );

INSERT INTO dbo.TwoRows
VALUES      (1),
            (2)

GO

CREATE VIEW V
WITH SCHEMABINDING
AS
  SELECT D.AddressId,
         D.CustomerId
  FROM   dbo.CustDefaultAddress D
         JOIN dbo.CustSecondaryAddress S
           ON D.AddressId = S.AddressId
              AND D.CustomerId = S.CustomerId
         CROSS JOIN dbo.TwoRows

GO

CREATE UNIQUE CLUSTERED INDEX IX
  ON V(AddressId, CustomerId)