SQL Server 缺省约束 示例

缺省约束

缺省约束经过定义列的缺省value或应用数据库的缺省value对象绑定表的列，来指定列的缺省value。SQL Server 推荐应用缺省约束，而不应用定义缺省value的方法来指定列的缺省value。有关绑定缺省约束的窍门请参见“数据完整性”章节。

定义缺省约束的语言规则如下：

CONSTRAINT constraint_title

DEFAULT constant_expression [FOR column_title]

例子：

constraint de_order_quantity default 100 for order_quantity

留意：不能在创建表时定义缺省约束，只能向已经创建好的表中添加缺省约束。

为既有表增加DEFAULT约束

ALTER TABLE table_name 

ADD CONSTRAINT constraint_name 

DEFAULT default_value 

FOR column_name

Example 1:

go 
IF  EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[dbo].[ABC_Table]') AND type in (N'U'))    
DROP TABLE [dbo].[ABC_Table]    
go    
SET ANSI_NULLS ON    
GO    
SET QUOTED_IDENTIFIER ON    
GO    
CREATE TABLE [dbo].[ABC_Table](    
    [Type_ID] [int] NOT NULL,    
    [Name] [nvarchar](100) NULL,    
    [CreatedBy] [nvarchar](50) NULL,    
    [CreatedOn] [datetime] NULL ,    
    [UpdatedBy] [nvarchar](50) NULL,    
    [UpdatedOn] [datetime] NULL,    
 CONSTRAINT [PK_ABC_Table] PRIMARY KEY CLUSTERED     
(    
    [Type_ID] 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 ALTER TABLE ABC_Table     
ADD CONSTRAINT DF_ABC_Table_CreatedBy      
DEFAULT 'Admin'  
FOR [CreatedBy]      
go       
ALTER TABLE ABC_Table      
ADD CONSTRAINT DF_ABC_Table_CreatedOn      
DEFAULT (getdate())      
FOR [CreatedOn]    
go

Example 2:

SET ANSI_NULLS ON   
GO    
SET QUOTED_IDENTIFIER ON    
GO    
CREATE TABLE [dbo].[ABC_Table](    
    [Type_ID] [int] NOT NULL,    
    [Name] [nvarchar](100) NULL,    
    [CreatedBy] [nvarchar](50) NULL CONSTRAINT [DF_ABC_Table_CreatedBy]  DEFAULT ('Admin'),    
    [CreatedOn] [datetime] NULL CONSTRAINT [DF_ABC_Table_CreatedOn]  DEFAULT (getdate()),    
    [UpdatedBy] [nvarchar](50) NULL,    
    [UpdatedOn] [datetime] NULL,    
 CONSTRAINT [PK_ABC_Table] PRIMARY KEY CLUSTERED     
(    
    [Type_ID] ASC    
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]    
) ON [PRIMARY]

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