CREATE TABLE aaa (
  x number primary key,
  y varchar2(10))

CREATE TABLE bbb (
  x number primary key,
  y varchar2(10))

insert into aaa values (1, 'foo')
insert into bbb values (1, 'bar')

insert into bbb values (2, 'bar')

1 foo bar
2 null bar

select b.x, b.y, a.y from bbb b
LEFT JOIN aaa a ON b.x = a.x