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

试试下面的命令;

ALTER TABLE Person11
ADD CONSTRAINT col_1_def
DEFAULT 'This is not NULL' FOR Address

不能使用alter列，而是使用add列

ALTER TABLE Employee 
ADD DEFAULT('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;

ALTER TABLE Employee ADD DEFAULT 'SANDNES' FOR CityBorn

这将在SQL Server工作:

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