如果这个表已经在数据库中创建了，那么你可以稍后通过使用下面的SQL查询添加一个唯一的约束:

ALTER TABLE dbo.User
  ADD CONSTRAINT ucCodes UNIQUE (fcode, scode, dcode)

通过在表创建时使用约束定义，可以指定一个或多个横跨多列的约束。从technet的文档中简化的语法形式为:

CONSTRAINT constraint_name UNIQUE [ CLUSTERED | NONCLUSTERED ] 
(
    column [ ASC | DESC ] [ ,...n ]
)

因此，结果表定义将是:

CREATE TABLE [dbo].[user](
    [userID] [int] IDENTITY(1,1) NOT NULL,
    [fcode] [int] NULL,
    [scode] [int] NULL,
    [dcode] [int] NULL,
    [name] [nvarchar](50) NULL,
    [address] [nvarchar](50) NULL,
    CONSTRAINT [PK_user_1] PRIMARY KEY CLUSTERED 
    (
        [userID] ASC
    ),
    CONSTRAINT [UQ_codes] UNIQUE NONCLUSTERED
    (
        [fcode], [scode], [dcode]
    )
) ON [PRIMARY]

如果这个表已经在数据库中创建了，那么你可以稍后通过使用下面的SQL查询添加一个唯一的约束:

ALTER TABLE dbo.User
  ADD CONSTRAINT ucCodes UNIQUE (fcode, scode, dcode)

USE [TSQL2012]
GO

/****** Object:  Table [dbo].[Table_1]    Script Date: 11/22/2015 12:45:47 PM ******/
SET ANSI_NULLS ON
GO

SET QUOTED_IDENTIFIER ON
GO

CREATE TABLE [dbo].[Table_1](
    [seq] [bigint] IDENTITY(1,1) NOT NULL,
    [ID] [int] NOT NULL,
    [name] [nvarchar](50) NULL,
    [cat] [nvarchar](50) NULL,
 CONSTRAINT [PK_Table_1] PRIMARY KEY CLUSTERED 
(
    [ID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY],
 CONSTRAINT [IX_Table_1] UNIQUE NONCLUSTERED 
(
    [name] ASC,
    [cat] 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

这也可以在GUI中完成。下面是一个向现有表添加多列唯一约束的示例。

在表下，右键单击“索引”->单击/悬停“新建索引”->单击“非聚集索引”…

将给出一个默认的索引名称，但您可能希望更改它。选中“惟一”复选框，然后单击“添加…”按钮

检查您希望包含的列

在每个窗口中单击OK，就完成了。