从下表中，我需要fNAME，lNAME，USER_MAIL和PARENT_USERNAME的分层视图，路径紧跟着lNAME / PARENT_USERNAME
create table USER (
USERNAME varchar2 (30),
fNAME varchar2 (30) not null,
lNAME varchar2 (30) not null,
USER_PHONE char(10) null,
USER_MAIL varchar2 (30) null,
PARENT_USERNAME varchar2 (30) null,
constraint USERNAME_PK primary key (USERNAME) );