错误定义外键

Question

我有以下两个数据库表中定义：

CREATE TABLE [dbo].[Classrooms] (
    [ID]    INT   IDENTITY (1, 1) NOT NULL, 
    [SystemAccount_ID] INT   NOT NULL, 
    [ClassroomName] VARCHAR (30) NOT NULL, 
    CONSTRAINT [PK_Table] PRIMARY KEY CLUSTERED ([ID]), 
    CONSTRAINT [FK_Classrooms_SystemAccount] FOREIGN KEY ([SystemAccount_ID]) REFERENCES [dbo].[SystemAccounts] ([ID]) 
); 

CREATE TABLE [dbo].[Students] (
    [ID]    INT   IDENTITY (1, 1) NOT NULL, 
    [SystemAccount_ID] INT   NOT NULL, 
    [Classroom_ID]  INT   NULL, 
    [PhotoID]   INT   NULL, 
    [FirstName]  VARCHAR (20) NOT NULL, 
    [LastName]   VARCHAR (40) NOT NULL, 
    [NewsTemplate]  TINYINT  NOT NULL, 
    CONSTRAINT [PK_Students] PRIMARY KEY CLUSTERED ([ID] ASC), 
    CONSTRAINT [FK_Students_Classrooms] FOREIGN KEY ([Classroom_ID]) REFERENCES [dbo].[Classrooms] ([ID]), 
    CONSTRAINT [FK_Students_SystemAccounts] FOREIGN KEY ([SystemAccount_ID]) REFERENCES [dbo].[SystemAccounts] ([ID]) 
); 

数据模型的细节：

• 学生通过Classroom_ID属于零或一个教室FK
• 学生属于一个系统帐户通过SystemAccount_ID FK
• 教室属于一个系统帐户通过SystemAccount_ID FK（暗示系统交流计数可以有零个或多个教室）

什么我试图做的是当学生们被添加到一个教室强制执行（通过设置在学生表中的Classroom_ID键）的教室属于同一系统帐户作为学生。我可以很容易地在业务逻辑层强制执行此操作，但是我会要求每个程序员都要记住这样做。所以理想情况下，我可以在数据层做到这一点作为一个约束。

我尝试添加一个FK约束学生表：

CONSTRAINT [FK_Students_ToTable] FOREIGN KEY ([SystemAccount_ID]) REFERENCES [Classrooms]([SystemAccount_ID]) 

导致的SQL Server以下错误称赞：

更新无法继续，由于验证错误。
请更正以下错误并重试。

SQL71516 ::引用表'[dbo]。[Classrooms]'不包含与外键中的引用列表匹配的主键或候选键。如果被引用的列是一个计算列，它应该被保留。

我已经尝试了几个不同的东西，但我的SQL mojo功能不足以破解这一个。任何帮助将不胜感激。

Answer 1

在两列的组合添加UNIQUE约束在Classrooms：

CREATE TABLE [dbo].[Classrooms] (
    [ID]    INT   IDENTITY (1, 1) NOT NULL, 
    [SystemAccount_ID] INT   NOT NULL, 
    [ClassroomName] VARCHAR (30) NOT NULL, 
    CONSTRAINT [PK_Table] 
    PRIMARY KEY CLUSTERED ([ID]), 
    CONSTRAINT [FK_Classrooms_SystemAccount] 
    FOREIGN KEY ([SystemAccount_ID]) 
    REFERENCES [dbo].[SystemAccounts] ([ID]), 
    CONSTRAINT [UQ_Classrooms_ID_SystemAccount_ID] 
    UNIQUE ([SystemAccount_ID], [ID]) 
); 

然后，在Students表，将两者结合起来FOREIGN KEY约束为一体，或在你的情况（因为Classroom_ID ISNULLABLE）改变FK到Classroom使用两列组合：

CREATE TABLE [dbo].[Students] (
    [ID]    INT   IDENTITY (1, 1) NOT NULL, 
    [SystemAccount_ID] INT   NOT NULL, 
    [Classroom_ID]  INT   NULL, 
    [PhotoID]   INT   NULL, 
    [FirstName]  VARCHAR (20) NOT NULL, 
    [LastName]   VARCHAR (40) NOT NULL, 
    [NewsTemplate]  TINYINT  NOT NULL, 
    CONSTRAINT [PK_Students] 
    PRIMARY KEY CLUSTERED ([ID] ASC), 
    CONSTRAINT [FK_Students_Classrooms] 
    FOREIGN KEY ([SystemAccount_ID], [Classroom_ID]) 
    REFERENCES [dbo].[Classrooms] ([SystemAccount_ID], [ID]), 
    CONSTRAINT [FK_Students_SystemAccounts]   -- this wouldn't be needed if 
    FOREIGN KEY ([SystemAccount_ID])    -- Classrooms_ID was NOT NULL 
    REFERENCES [dbo].[SystemAccounts] ([ID]) 
);