-- Database schema
| artist : Artist_ID [ INT ] primary_key , Name [ TEXT ] , Country [ TEXT ] , Year_Join [ INT ] , Age [ INT ] | exhibition : Exhibition_ID [ INT ] primary_key , Year [ INT ] , Theme [ TEXT ] , Artist_ID [ INT ] exhibition.Artist_ID = artist.Artist_ID , Ticket_Price [ INT ] | exhibition_record : Exhibition_ID [ INT ] primary_key exhibition_record.Exhibition_ID = exhibition.Exhibition_ID , Date [ TEXT ] , Attendance [ INT ] |
-- -- Which themes have had corresponding exhibitions that have had attendance both below 100 and above 500?

SELECT T2.theme FROM exhibition_record AS T1 JOIN exhibition AS T2 ON T1.exhibition_id  =  T2.exhibition_id WHERE T1.attendance  <  100 INTERSECT SELECT T2.theme FROM exhibition_record AS T1 JOIN exhibition AS T2 ON T1.exhibition_id  =  T2.exhibition_id WHERE T1.attendance  >  500;