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);
