-- Database schema
| cites : cited_paper_id [ TEXT ] , citing_paper_id [ TEXT ] | paper : paper_id [ TEXT ] primary_key , class_label [ TEXT ] | content : paper_id [ TEXT ] content.paper_id = paper.paper_id , word_cited_id [ TEXT ] |
-- -- Among all the DB class type citation, which word is the most frequently cited?

SELECT T2.word_cited_id FROM paper AS T1 INNER JOIN content AS T2 ON T1.paper_id = T2.paper_id WHERE T1.class_label = 'DB' GROUP BY T2.word_cited_id ORDER BY COUNT(T2.word_cited_id) DESC LIMIT 1;