Among all the DB class type citation, which word is the most frequently cited?
class type refers to class_label; class_label = 'DB'; word that is most frequently cited refers to max(count(word_cited_id);
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