CREATE TABLE exhibition (exhibition_id DECIMAL, year DECIMAL, theme TEXT, artist_id DECIMAL, ticket_price DECIMAL)
CREATE TABLE artist (artist_id DECIMAL, name TEXT, country TEXT, year_join DECIMAL, age DECIMAL)
CREATE TABLE exhibition_record (exhibition_id DECIMAL, date TEXT, attendance DECIMAL)

Show the theme for exhibitions with both records of an attendance 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
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