with t
as
(
 select 'naver.com,gmail.com,hanmail.net' email_addr from dual union all
 select 'gmail.com,hanmail.net' from dual union all
 select 'naver.com' from dual union all
 
 select '' from dual
)
select *
from t
where email_addr != 'naver.com'

with t
as
(
 select 'aaa@naver.com,aaa@gmail.com,aaa@hanmail.net' email_addr from dual union all
 select 'aaa@gmail.com,aaa@hanmail.net' from dual union all
 select 'aaa@naver.com' from dual union all
 
 select 'naver.com,gmail.com,hanmail.net' email_addr from dual union all
 select 'gmail.com,hanmail.net' from dual union all
 select 'naver.com' from dual union all
 
 select '' from dual union all
 
 select 'aaa@naver.com,naver.com,bbb@naver.com' from dual
)
select *
from t a
where regexp_replace(email_addr,'[^,]*naver.com|,') is not null