SELECT CategoryID, AVG(Price)
FROM Products
WHERE CategoryID="3"
GROUP BY CategoryID

CategoryID  AVG(Price)
3           25.16

SELECT CategoryID, AVG(Price)
FROM Products
WHERE Price > 30
GROUP BY CategoryID;

CategoryID  AVG(Price)
1           154.75
2           41.95
3           51.3575
4           38.300000000000004
5           35.625
6           73.14750000000001
7           49.3
8           46.75

SELECT CategoryID, AVG(Price)
FROM Products
WHERE CategoryID="3"
GROUP BY CategoryID
having AVG(Price)>x