第一次掉落约束

https://stackoverflow.com/a/49393045/2547164

DECLARE @ConstraintName nvarchar(200)
SELECT @ConstraintName = Name FROM SYS.DEFAULT_CONSTRAINTS
WHERE PARENT_OBJECT_ID = OBJECT_ID('__TableName__')
AND PARENT_COLUMN_ID = (SELECT column_id FROM sys.columns
                        WHERE NAME = N'__ColumnName__'
                        AND object_id = OBJECT_ID(N'__TableName__'))
IF @ConstraintName IS NOT NULL
EXEC('ALTER TABLE __TableName__ DROP CONSTRAINT ' + @ConstraintName)

第二步创建默认值

ALTER TABLE [table name] ADD DEFAULT [default value] FOR [column name]

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

ALTER TABLE Employee ADD DEFAULT 'SANDNES' FOR CityBorn

Hoodaticus的解决方案是完美的，谢谢，但我也需要它是可重新运行的，并找到了这种方式来检查它是否已经完成……

IF EXISTS(SELECT * FROM information_schema.columns 
           WHERE table_name='myTable' AND column_name='myColumn' 
             AND Table_schema='myDBO' AND column_default IS NULL) 
BEGIN 
  ALTER TABLE [myDBO].[myTable] ADD DEFAULT 0 FOR [myColumn] --Hoodaticus
END

您可以使用以下语法，有关详细信息，请参阅此问题和答案:将具有默认值的列添加到SQL Server中的现有表中

语法:

ALTER TABLE {TABLENAME} 
ADD {COLUMNNAME} {TYPE} {NULL|NOT NULL} 
CONSTRAINT {CONSTRAINT_NAME} DEFAULT {DEFAULT_VALUE}
WITH VALUES

例子:

ALTER TABLE SomeTable
ADD SomeCol Bit NULL --Or NOT NULL.
CONSTRAINT D_SomeTable_SomeCol --When Omitted a Default-Constraint Name is 
autogenerated.
DEFAULT (0)--Optional Default-Constraint.
WITH VALUES --Add if Column is Nullable and you want the Default Value for Existing Records.

另一种方法:

右键单击表并单击“设计”，然后单击要设置默认值的列。

然后在页面底部添加一个默认值或绑定:例如字符串为'1'或int为1。

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