按属性和计数分组，postgreSQL

Question

我编写了以下代码，用于计算soldBooks表中每个book_id的实例数。

    SELECT book_id, sum(counter) AS no_of_books_sold, sum(retail_price) AS generated_revenue
    FROM(
        SELECT  book_id,1 AS counter, retail_price
        FROM shipments 
        LEFT JOIN editions ON (shipments.isbn = editions.isbn)
        LEFT JOIN stock ON (shipments.isbn = stock.isbn)
        ) AS soldBooks
    GROUP BY book_id

正如你所看到的，我用了一个＆＃34;计数器＆＃34;为了解决我的问题。但我相信必须有一个更好，更内置的方法来实现相同的结果！必须有某种方法通过给定属性将表组合在一起，并创建一个显示EACH属性计数的新列。有人可以跟我分享一下吗？

谢谢！

Answer 1

SELECT book_id, 
       COUNT(book_id) AS no_books_sold, 
       SUM(retail_price) AS gen_rev
  FROM shipments 
         JOIN editions ON (shipments.isbn=editions.isbn)
         JOIN stock ON (shipments.isbn=stock.isbn)
 GROUP BY book_id