クラスの本のコストに関するMySQLクエリ

これを試してください：

1行で：

SELECT SUM(Class.Students * Books.Cost) AS BookCost  FROM Books INNER JOIN Class       ON Books.ClassId = Class.ClassId WHERE Books.ClassId = <CLASS-ID-VALUE>  GROUP BY Books.ClassId

フォーマットあり：

SELECT SUM(Class.Students * Books.Cost) AS BookCost
  FROM Books INNER JOIN Class
   ON Books.ClassId = Class.ClassId
WHERE Books.ClassId = <CLASS-ID-VALUE>
GROUP BY Books.ClassId