Oracle 如何识别左联接中的空列与空行

select tab1.usr, tab2.salary 
from tab1
LEFT JOIN tab2 ON (tab1.usr = tab2.usr and tab1.name = tab2.name);

USR   salary
-----   -------
1111  5001
2222  NULL
3333  NULL

Since usr: 2222 not exists in tab2 I am trying to get the output like below:
USR   salary
-----   -------
1111  5001
2222  NULL
3333  'NOT FOUND'

create table tab1 
( 
usr number, 
name varchar2(10) 
);

create table tab2 
( 
usr number, 
name varchar2(10), 
salary number 
);

insert into tab1 
values (1111, 'Jerry');

insert into tab1 
values (2222, 'John');

insert into tab1 
values (3333, 'Scott');

insert into tab2 
values (1111, 'Jerry', 5001);

insert into tab2 
values (3333, 'Jerry', NULL);

commit;

select tab1.usr, tab2.usr ,
       case
         when tab2.usr is not null then 
           to_char(tab2.salary)
         else
           'NOT FOUND'
       end case_sal,
       decode( tab2.usr , null , 'NOT FOUND', to_char(tab2.salary) ) dec_sal
from   tab1
left join tab2 
on     tab1.usr = tab2.usr;

USR   USR   CASE_SAL   DEC_SAL    
1111  1111  5001       5001       
3333  3333                        
2222        NOT FOUND  NOT FOUND