语法:

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

示例:

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.

注意事项:

可选约束名称:
如果您省略 CONSTRAINT D_SomeTable_SomeCol，则 SQL Server 将自动生成
  具有有趣名称的默认约束，例如:DF__SomeTa__SomeC__4FB7FEF6

Notes:

Optional Constraint Name:
If you leave out CONSTRAINT D_SomeTable_SomeCol then SQL Server will autogenerate
    a Default-Contraint with a funny Name like: DF__SomeTa__SomeC__4FB7FEF6

可选的 With-Values 声明:
WITH VALUES 仅在您的列为 Nullable 时才需要
  并且您想要用于现有记录的默认值.
如果您的列是 NOT NULL，那么它将自动使用默认值
  对于所有现有记录，无论您是否指定 WITH VALUES.

Optional With-Values Statement:
The WITH VALUES is only needed when your Column is Nullable
    and you want the Default Value used for Existing Records.
If your Column is NOT NULL, then it will automatically use the Default Value
    for all Existing Records, whether you specify WITH VALUES or not.

插入如何与默认约束一起工作:
如果你在SomeTable中插入一个Record并且不指定SomeCol的值，那么它会默认为0代码>.
如果您插入记录 和 将 SomeCol 的值指定为 NULL(并且您的列允许空值)，
  那么默认约束将不被使用，NULL将被插入作为值.

How Inserts work with a Default-Constraint:
If you insert a Record into SomeTable and do not Specify SomeCol's value, then it will Default to 0.
If you insert a Record and Specify SomeCol's value as NULL (and your column allows nulls),
    then the Default-Constraint will not be used and NULL will be inserted as the Value.

注释是基于每个人在下面的很好的反馈.
特别鸣谢:
  @Yatrix、@WalterStabosz、@YahooSerious 和 @StackMan 发表评论.

Notes were based on everyone's great feedback below.
Special Thanks to:
    @Yatrix, @WalterStabosz, @YahooSerious, and @StackMan for their Comments.