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;

刚刚发现3个简单的步骤来改变已经存在的列，之前是空的

update   orders
set BasicHours=0 where BasicHours is null

alter table orders 
add default(0) for BasicHours

alter table orders 
alter  column CleanBasicHours decimal(7,2) not null 

ALTER TABLE [dbo].[Employee] ADD  DEFAULT ('N') FOR [CityBorn]

这将在SQL Server工作:

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

ALTER TABLE Employee ADD DEFAULT 'SANDNES' FOR CityBorn