Show the theme of each exhibition.
select t2.theme from exhibition_record as t1 join exhibition as t2 on t1.exhibition_id = t2.exhibition_id
How about of each exhibition with 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
How about of each exhibition with an attendance below 100 or 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