示例
create table [user](
[id] int identity primary key not null
[username] varchar (20)
)
create table [user_photo] (
[id] int identity primary key not null
[uid] int reference [user]([id]) on delete cascade not for replication not null
--此处就建立了[user_photo]与[user]的关系,外键约束(和级联删除一样)
[filename] varchar(30)
)