錯誤定義外鍵

Question

我有以下兩個數據庫表中定義：

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]) 
); 

數據模型的細節：

• 學生通過Classroom_ID屬於零或一個教室FK
• 學生屬於一個系統帳戶通過SystemAccount_ID FK
• 教室屬於一個系統帳戶通過SystemAccount_ID FK（暗示系統交流計數可以有零個或多個教室）

什麼我試圖做的是當學生們被添加到一個教室強制執行（通過設置在學生表中的Classroom_ID鍵）的教室屬於同一系統帳戶作爲學生。我可以很容易地在業務邏輯層強制執行此操作，但是我會要求每個程序員都要記住這樣做。所以理想情況下，我可以在數據層做到這一點作爲一個約束。

我嘗試添加一個FK約束學生表：

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

導致的SQL Server以下錯誤稱讚：

更新無法繼續，由於驗證錯誤。
請更正以下錯誤並重試。

SQL71516 ::引用表'[dbo]。[Classrooms]'不包含與外鍵中的引用列表匹配的主鍵或候選鍵。如果被引用的列是一個計算列，它應該被保留。

我已經嘗試了幾個不同的東西，但我的SQL mojo功能不足以破解這一個。任何幫助將不勝感激。

Answer 1

在兩列的組合添加UNIQUE約束在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]) 
); 

然後，在Students表，將兩者結合起來FOREIGN KEY約束爲一體，或在你的情況（因爲Classroom_ID ISNULLABLE）改變FK到Classroom使用兩列組合：

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]) 
);