FROM exam_knowledge INNER JOIN exam_database ON exam_knowledge.knowledgeid = exam_database.knowledgeid
WHERE (((exam_knowledge.H_YsnSelected)=True) AND ((exam_database.difficulty)="难")) OR (((exam_knowledge.L_YsnSelected)=True) AND ((exam_database.difficulty)="易")) OR (((exam_knowledge.M_YsnSelected)=True) AND ((exam_database.difficulty)="中"))