What is the average of superheroes with no skin colour?
average = DIVIDE(COUNT(superhero.id), SUM(skin_colour_id = 1)); no skin colour refers to skin_colour_id WHERE colour.id = 1;
SELECT CAST(COUNT(*) AS REAL) / SUM(CASE WHEN T2.id = 1 THEN 1 ELSE 0 END) FROM superhero AS T1 INNER JOIN colour AS T2 ON T1.skin_colour_id = T2.id