What is the average number of characters in all the works of Shakespeare?
average number = divide(sum(character_id), count(work_id))
SELECT SUM(DISTINCT T4.id) / COUNT(T1.id) FROM works AS T1 INNER JOIN chapters AS T2 ON T1.id = T2.work_id INNER JOIN paragraphs AS T3 ON T2.id = T3.chapter_id INNER JOIN characters AS T4 ON T3.character_id = T4.id