将不同表中的多列作为一列的外键

Question

我对将多个列作为外键指向单个列存在疑问。让我用一个例子来解释。

例如：我有3个表：

公司1：Company_ID(guid) as Primary key, Address (varchar(25))

Company2的：Company_ID(guid) as Primary key, Address (varchar(25))

员工：Employee_ID(guid) as Primary key, Company_ID (Foreign key referencing from Company1 and Company2)

这里公司的外键Employee表中的_ID来自2个不同的表。 这工作在SQL服务器2008.

我的问题是我不知何故觉得这是不正确的。我从来没有在任何系统中做过这件事，但我正在尝试这个样本，它的工作原理。我无法解释自己这里有什么不对，因为它完美地工作。

我的假设是否正确或请引导我关于此？上述关系中的缺陷是什么？

USE [master] 
GO 
/****** Object: Database [Test] Script Date: 08/07/2012 14:32:32 ******/ 
CREATE DATABASE [Test] ON PRIMARY 
(NAME = N'Test', FILENAME = N'C:\Program Files\Microsoft SQL Server\MSSQL10_50.FLEXIQUOTE\MSSQL\DATA\Test.mdf' , SIZE = 3072KB , MAXSIZE = UNLIMITED, FILEGROWTH = 1024KB) 
LOG ON 
(NAME = N'Test_log', FILENAME = N'C:\Program Files\Microsoft SQL Server\MSSQL10_50.FLEXIQUOTE\MSSQL\DATA\Test_log.ldf' , SIZE = 1024KB , MAXSIZE = 2048GB , FILEGROWTH = 10%) 
GO 
ALTER DATABASE [Test] SET COMPATIBILITY_LEVEL = 100 
GO 
IF (1 = FULLTEXTSERVICEPROPERTY('IsFullTextInstalled')) 
begin 
EXEC [Test].[dbo].[sp_fulltext_database] @action = 'enable' 
end 
GO 
ALTER DATABASE [Test] SET ANSI_NULL_DEFAULT OFF 
GO 
ALTER DATABASE [Test] SET ANSI_NULLS OFF 
GO 
ALTER DATABASE [Test] SET ANSI_PADDING OFF 
GO 
ALTER DATABASE [Test] SET ANSI_WARNINGS OFF 
GO 
ALTER DATABASE [Test] SET ARITHABORT OFF 
GO 
ALTER DATABASE [Test] SET AUTO_CLOSE OFF 
GO 
ALTER DATABASE [Test] SET AUTO_CREATE_STATISTICS ON 
GO 
ALTER DATABASE [Test] SET AUTO_SHRINK OFF 
GO 
ALTER DATABASE [Test] SET AUTO_UPDATE_STATISTICS ON 
GO 
ALTER DATABASE [Test] SET CURSOR_CLOSE_ON_COMMIT OFF 
GO 
ALTER DATABASE [Test] SET CURSOR_DEFAULT GLOBAL 
GO 
ALTER DATABASE [Test] SET CONCAT_NULL_YIELDS_NULL OFF 
GO 
ALTER DATABASE [Test] SET NUMERIC_ROUNDABORT OFF 
GO 
ALTER DATABASE [Test] SET QUOTED_IDENTIFIER OFF 
GO 
ALTER DATABASE [Test] SET RECURSIVE_TRIGGERS OFF 
GO 
ALTER DATABASE [Test] SET DISABLE_BROKER 
GO 
ALTER DATABASE [Test] SET AUTO_UPDATE_STATISTICS_ASYNC OFF 
GO 
ALTER DATABASE [Test] SET DATE_CORRELATION_OPTIMIZATION OFF 
GO 
ALTER DATABASE [Test] SET TRUSTWORTHY OFF 
GO 
ALTER DATABASE [Test] SET ALLOW_SNAPSHOT_ISOLATION OFF 
GO 
ALTER DATABASE [Test] SET PARAMETERIZATION SIMPLE 
GO 
ALTER DATABASE [Test] SET READ_COMMITTED_SNAPSHOT OFF 
GO 
ALTER DATABASE [Test] SET HONOR_BROKER_PRIORITY OFF 
GO 
ALTER DATABASE [Test] SET READ_WRITE 
GO 
ALTER DATABASE [Test] SET RECOVERY SIMPLE 
GO 
ALTER DATABASE [Test] SET MULTI_USER 
GO 
ALTER DATABASE [Test] SET PAGE_VERIFY CHECKSUM 
GO 
ALTER DATABASE [Test] SET DB_CHAINING OFF 
GO 
USE [Test] 
GO 
/****** Object: Table [dbo].[Table2] Script Date: 08/07/2012 14:32:32 ******/ 
SET ANSI_NULLS ON 
GO 
SET QUOTED_IDENTIFIER ON 
GO 
CREATE TABLE [dbo].[Table2](
    [Company2ID] [int] NOT NULL, 
    [COmpanyAddress] [nchar](10) NULL, 
CONSTRAINT [PK_Table2] PRIMARY KEY CLUSTERED 
(
    [Company2ID] ASC 
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY] 
) ON [PRIMARY] 
GO 
/****** Object: Table [dbo].[Table1] Script Date: 08/07/2012 14:32:32 ******/ 
SET ANSI_NULLS ON 
GO 
SET QUOTED_IDENTIFIER ON 
GO 
CREATE TABLE [dbo].[Table1](
    [Company1ID] [int] NOT NULL, 
    [CompanyName] [nchar](10) NULL, 
CONSTRAINT [PK_Table1] PRIMARY KEY CLUSTERED 
(
    [Company1ID] ASC 
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY] 
) ON [PRIMARY] 
GO 
/****** Object: Table [dbo].[Table3] Script Date: 08/07/2012 14:32:32 ******/ 
SET ANSI_NULLS ON 
GO 
SET QUOTED_IDENTIFIER ON 
GO 
CREATE TABLE [dbo].[Table3](
    [EMployeeID] [int] NOT NULL, 
    [Company] [int] NULL, 
CONSTRAINT [PK_Table3] PRIMARY KEY CLUSTERED 
(
    [EMployeeID] ASC 
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY] 
) ON [PRIMARY] 
GO 
/****** Object: ForeignKey [FK_Table3_Table1] Script Date: 08/07/2012 14:32:32 ******/ 
ALTER TABLE [dbo].[Table3] WITH CHECK ADD CONSTRAINT [FK_Table3_Table1] FOREIGN KEY([Company]) 
REFERENCES [dbo].[Table1] ([Company1ID]) 
ON UPDATE CASCADE 
ON DELETE CASCADE 
GO 
ALTER TABLE [dbo].[Table3] CHECK CONSTRAINT [FK_Table3_Table1] 
GO 
/****** Object: ForeignKey [FK_Table3_Table2] Script Date: 08/07/2012 14:32:32 ******/ 
ALTER TABLE [dbo].[Table3] WITH CHECK ADD CONSTRAINT [FK_Table3_Table2] FOREIGN KEY([Company]) 
REFERENCES [dbo].[Table2] ([Company2ID]) 
ON UPDATE CASCADE 
ON DELETE CASCADE 
GO 
ALTER TABLE [dbo].[Table3] CHECK CONSTRAINT [FK_Table3_Table2] 
GO 

Answer 1

你的感觉是对的。 。 。这不是“正确的”。或者，至少，这是一个糟糕的设计决定。

但是，您的外键关系可能是一个复合键。一般来说，你会有另一列指定哪个表引用。复杂的是，这可能是一个“隐含的”列。所以，如果一个表有美国地址和其他非美国地址，那么这两个表都不会有“IsUS”标志。但是，美国的记录会提到一张桌子，而非美国的则是指向另一张桌子。

另一个问题是如何维护这些密钥。你真的不想有两个具有相互依赖的主键的表。解决这个问题的典型方法是将两个表合并为一个。

虽然这是一个糟糕的主意，但您可能不得不忍受这种结构。如果你可以改变它，你应该。