Show the theme for exhibitions with records of an attendance below 100
SELECT T2.theme FROM exhibition_record AS T1 JOIN exhibition AS T2 ON T1.exhibition_id   =   T2.exhibition_id WHERE T1.attendance  <  100
theme for exhibitions with attendance below 100 refers to SELECT theme FROM exhibition WHERE attendance < 100; JOIN operation is used to link exhibition_record with exhibition on the condition that their exhibition_id matches;