如何阻止数据重复从多个表sql调用的数据

Question

我正在尝试打印以下列isbn，title，fName，surname和quantity，但由于有多个订单，数据打印出来的每个订单，而不是只有一个订单数量的数字。我该怎么做？

SELECT book.isbn, book.title, author.fName, author.surname, orders.quantity
FROM book, author, orders
WHERE orders.bookID = book.isbn
ORDER BY book.isbn DESC;

Answer 1

您需要GROUP BY条款。我还建议添加显式连接（并删除现在多余的WHERE子句）：

SELECT book.isbn, book.title, author.fName, author.surname, SUM(orders.quantity) AS TotalOrders
FROM (book INNER JOIN author ON book.authorID = author.authorID)
  INNER JOIN orders ON book.isbn = orders.bookID
GROUP BY book.isbn, book.title, author.fName, author.surname
ORDER BY book.isbn DESC

Answer 2

如果您想要每个订单的数量，您的查询将为每个订单返回一行。如果您使用某种应用程序来显示结果，那么您可以使用应用程序代码来解决问题。

另一方面，如果您只是想知道订购了多少本书，那么您的查询将变为：

SELECT book.isbn, book.title, author.fName, author.surname, sum(orders.quantity) total
FROM book, author, orders
WHERE orders.bookID = book.isbn\
group by book.isbn, book.title, author.fName, author.surname
ORDER BY book.isbn DESC