Aggregate two columns

I have two table and I want to merge them

TERMS_TABLE
ID | TERMS
309 | 'hardware'
309 | 'software'
309 | 'computer'


TFIDF_TABLE
ID | TERMS
309 |'computer,phone,mp3....'


Now I want to add TERMS column of TERMS_TABLE to terms column of TFIDF_TABLE but If TFIDF_TABLE already contains TERMS of TERMS_TABLE then I should not insert this term to the NEW_TFIDF_TABLE , like that

result should be:

NEW_TFIDF_TABLE
ID | TERMS
309 |'computer,phone,mp3....,hardware,software'

How can I do that ?

try like this...

select
deptno,
rtrim (xmlagg (xmlelement (e, ename || ',')).extract ('//text()'), ',') enames
from
emp
group by
deptno
;