ALTER TABLE tblUser
 ADD CONSTRAINT DF_User_CreatedON DEFAULT GETDATE() FOR CreatedOn

正确的做法如下:

执行如下命令: Sp_help[表名] 复制约束的名称。 删除默认约束: ALTER TABLE[表名]DROP[约束名] 运行如下命令: 修改表[表名]为[列名]添加默认值[默认值]

这将在SQL Server工作:

ALTER TABLE Employee ADD CONSTRAINT DF_SomeName DEFAULT N'SANDNES' FOR CityBorn;

如果一个限制已经存在，它的默认名称是:

-- Drop existing default constraint on Employee.CityBorn
DECLARE @default_name varchar(256);
SELECT @default_name = [name] FROM sys.default_constraints WHERE parent_object_id=OBJECT_ID('Employee') AND COL_NAME(parent_object_id, parent_column_id)='CityBorn';
EXEC('ALTER TABLE Employee DROP CONSTRAINT ' + @default_name);

-- Add default constraint on Employee.CityBorn
ALTER TABLE Employee ADD CONSTRAINT df_employee_1 DEFAULT 'SANDNES' FOR CityBorn;

就像Yuck的答案，检查允许脚本运行多次而没有错误。(与使用information_schema.columns相比，代码/自定义字符串更少)

IF object_id('DF_SomeName', 'D') IS NULL BEGIN
    Print 'Creating Constraint DF_SomeName'
   ALTER TABLE Employee ADD CONSTRAINT DF_SomeName DEFAULT N'SANDNES' FOR CityBorn;
END

ALTER TABLE Employee ADD DEFAULT 'SANDNES' FOR CityBorn