oracle中空值与空值之间应该是不想等的，那么有唯一性约束在字段是否可以有多个null之？

create table typeinfo(

typeid varchar2(10) primary key,

typename varchar2(20));

create table userinfo_f(

id varchar2(10) primary key,

username varchar2(20),

typeid_new varchar2(10) ,

constraint fk_typeid_new foreign key(typeid_new) references typeinfo(typeid));

insert into typeinfo values(1,'aaa',1);

insert into userinfo_f values(2,'bbb',null);

insert into userinfo_f values(3,'ccc',null);