SQL Server的唯一鍵和唯一索引會將空值（NULL）也算作重復值

我們先在SQL Server數據庫中，建立一張Students表：

CREATE TABLE [dbo].[Students](
    [ID] [int] IDENTITY(1,1) NOT NULL,
    [StudentCode] [nvarchar](50) NULL,
    [Name] [nvarchar](50) NULL,
    [Age] [int] NULL,
    [Sex] [nvarchar](5) NULL,
 CONSTRAINT [PK_Students] 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_UniqueStudentCode] UNIQUE NONCLUSTERED 
(
    [StudentCode] 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

該表的主鍵是列ID，該表的唯一鍵IX_UniqueStudentCode要求列StudentCode不能有重復值，但是列StudentCode可以為空值（NULL）。

 

首先我們插入四條數據到Students表中：

INSERT INTO [dbo].[Students]([StudentCode],[Name],[Age],[Sex])
VALUES
(N'S001',N'Jim',15,N'M'),
(N'S002',N'Tom',16,N'F'),
(N'S003',N'Bill',15,N'M'),
(NULL,N'Jerry',15,N'M')

可以看到我們在最后第四條數據中，給列StudentCode插入了空值（NULL）。

 

接着我們再插入一條列StudentCode為空值（NULL）的數據到Students表：

INSERT INTO [dbo].[Students]([StudentCode],[Name],[Age],[Sex])
VALUES
(NULL,N'Clark',16,N'M')

會發現SQL Server報錯，錯誤如下：

Msg 2627, Level 14, State 1, Line 1
Violation of UNIQUE KEY constraint 'IX_UniqueStudentCode'. Cannot insert duplicate key in object 'dbo.Students'. The duplicate key value is (<NULL>).
The statement has been terminated.

從錯誤中，我們可以看到，我們插入的數據違反了唯一鍵約束IX_UniqueStudentCode，插入了重復的空值（NULL）到Students表。

 

所以SQL Server的唯一鍵和唯一索引會將空值（NULL）也算作重復值，我們可以將上面的唯一鍵IX_UniqueStudentCode刪掉，將其建立為唯一索引：

ALTER TABLE [dbo].[Students] DROP  CONSTRAINT [IX_UniqueStudentCode]

CREATE UNIQUE NONCLUSTERED INDEX [IX_UniqueStudentCode] ON [dbo].[Students]
(
    [StudentCode] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO

會得到相同的實驗結果。