Ces questions permettent de vous situer vis à vis de la programmation et de l'informatique en général. Il est normal de ne pas tout connaître ;)
SELECT Marque, AVG(Prix)
FROM Produits
GROUP BY Marque
FWIW: I (@rondy) am not the creator of the content shared here, which is an excerpt from Edmond Lau's book. I simply copied and pasted it from another location and saved it as a personal note, before it gained popularity on news.ycombinator.com. Unfortunately, I cannot recall the exact origin of the original source, nor was I able to find the author's name, so I am can't provide the appropriate credits.