Ambiguity in Left joins (oracle only?)

select PTNO,PTNM,CATCD
from PARTS
left join CATEGORIES on (CATEGORIES.CATCD=PARTS.CATCD);

select PTNO,PTNM,PARTS.CATCD
from PARTS
left join CATEGORIES on (CATEGORIES.CATCD=PARTS.CATCD);

CREATE TABLE PARTS (PTNO NUMBER, CATCD NUMBER, SECCD NUMBER);

CREATE TABLE CATEGORIES(CATCD NUMBER);

CREATE TABLE SECTIONS(SECCD NUMBER, CATCD NUMBER);


select PTNO,CATCD
from PARTS
left join CATEGORIES on (CATEGORIES.CATCD=PARTS.CATCD)
left join SECTIONS on (SECTIONS.SECCD=PARTS.SECCD) ;

create table parts (ptno number , ptnm number , catcd number);
create table CATEGORIES (catcd number);

select PTNO,PTNM,CATCD from PARTS
left join CATEGORIES on (CATEGORIES.CATCD=PARTS.CATCD);

select id
from sysobjects s
left join syscolumns c on s.id = c.id

select id
from sysobjects
left join syscolumns  on sysobjects.id = syscolumns.id

select ptno, catcd
from parts, categories, sections
where categories.catcd (+) = parts.catcd
  and sections.seccd (+) = parts.seccd

select ptno, catcd
from (
  select ptno, seccd, catcd
  from parts
  left join categories on (categories.CATCD=parts.CATCD)
)
left join sections on (sections.SECCD=parts.SECCD)