how many albums does AC/DC have?
SELECT COUNT ( * )  FROM ALBUM AS T1 JOIN ARTIST AS T2 ON T1.ArtistId  =  T2.ArtistId WHERE T2.Name  =  "AC/DC"
counting albums by a specific artist (AC/DC) involves joining the Album table with the Artist table on a common key (ArtistId) and filtering by artist name (AC/DC).