ALTER TABLE Employee ADD DEFAULT 'SANDNES' FOR CityBorn

不能使用alter列，而是使用add列

ALTER TABLE Employee 
ADD DEFAULT('SANDNES') FOR CityBorn

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

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