18. 펭귄 날개와 몸무게의 상관 계수

SELECT
species,
ROUND(
	(
		SUM(flipper_length_mm * body_mass_g)
		- ( (SUM(flipper_length_mm) * SUM(body_mass_g)) / COUNT(*) )
	)
	/
	SQRT(
		(SUM(flipper_length_mm * flipper_length_mm) - ( (SUM(flipper_length_mm)*SUM(flipper_length_mm)) / COUNT(*) ))
		*
		(SUM(body_mass_g * body_mass_g) - ( (SUM(body_mass_g)*SUM(body_mass_g)) / COUNT(*) ))
), 3) AS corr
FROM penguins
WHERE flipper_length_mm IS NOT NULL
AND body_mass_g IS NOT NULL
GROUP BY species;