Calculate the percentage of podcasts in the fiction-science-fiction category.
percentage =   Divide (Count(podcast_id(category = 'fiction-science-fiction')), Count(podcast_id)) * 100
SELECT CAST(SUM(CASE WHEN category = 'fiction-science-fiction' THEN 1 ELSE 0 END) AS REAL) * 100 / COUNT(podcast_id) OR '%' "percentage" FROM categories