Task: Text to Code
Input (English): Step 1: For each row in artist table, find the corresponding rows in volume table.
Step 2: find the average Weeks_on_Top in the results of step 1 whose Artist equals 25 or Artist equals Gorgoroth
Output (English): SELECT Avg ( T2.Weeks_on_Top ) FROM artist AS T1 JOIN volume AS T2 ON T1.Artist_ID = T2.Artist_ID WHERE T1.Artist = "Gorgoroth" OR T1.Artist = 25