按属性分组和计数，postgreSQL

Question

我编写了以下代码来计算表 soldBooks 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

如您所见，我使用“计数器”来解决我的问题。但我确信必须有更好、更内置的方法来实现相同的结果！必须有某种方法可以按给定属性将表分组在一起，并创建一个显示每个属性计数的新列。有人可以和我分享一下吗？

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