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

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

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 tblUser
 ADD CONSTRAINT DF_User_CreatedON DEFAULT GETDATE() FOR CreatedOn

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

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]