Use a subquery in the where clause. For a delete query requirig a join, this example will delete rows that are unmatched in the joined table "docx_document" and that have a create date > 120 days in the "docs_documents" table.
delete from docs_documents d
where d.id in (
select a.id from docs_documents a
left join docx_document b on b.id = a.document_id
where b.id is null
and floor(sysdate - a.create_date) > 120
);