如何计算表中每个外键ID的实例数量？

这是我简单的SQL问题

我有两个表格：

图书

------------------------------------------------------- | book_id | author | genre | price | publication_date | ------------------------------------------------------- 

命令

 ------------------------------------ | order_id | customer_id | book_id | ------------------------------------ 

我想创build一个查询返回：

 -------------------------------------------------------------------------- | book_id | author | genre | price | publication_date | number_of_orders | -------------------------------------------------------------------------- 

换句话说，返回Books表中所有行的每一列，以及一个名为“number_of_orders”的计算列，该列计算每本书出现在Orders表中的次数。 （如果在订单表中没有出现书籍，该书应该列在结果集中，但“number_of_orders”应该为零 。

到目前为止，我已经想出了这个：

 SELECT books.book_id, books.author, books.genre, books.price, books.publication_date, count(*) as number_of_orders from books left join orders on (books.book_id = orders.book_id) group by books.book_id, books.author, books.genre, books.price, books.publication_date 

这几乎是正确的，但并不完全，因为即使从未在Orders表中列出书籍，“number_of_orders”也将为1。 而且，由于缺乏SQL知识，我相信这个查询是非常低效的。

什么是写这个查询的正确方法？ （对于它的价值，这需要在MySQL上工作，所以我不能使用任何其他供应商特定的function）。

提前致谢！

Answer 1

你的查询几乎是正确的，这是做到这一点（和最有效的）

 SELECT books.*, count(orders.book_id) as number_of_orders from books left join orders on (books.book_id = orders.book_id) group by books.book_id 

COUNT(*)可以在计数中包含NULL值，因为它计算所有行，而COUNT(orders.book_id)不会因为它忽略给定字段中的NULL值。

Answer 2

将count(*)更改为count(orders.book_id)

Answer 3

 SELECT b.book_id, b.author, b.genre, b.price, b.publication_date, coalesce(oc.Count, 0) as number_of_orders from books b left join ( select book_id, count(*) as Count from Order group by book_id ) oc on (b.book_id = oc.book_id) 

Answer 4

你在计算错误的东西。 你想要计算非空book_id的。

 SELECT books.book_id, books.author, books.genre, books.price, books.publication_date, count(orders.book_id) as number_of_orders from books left join orders on (books.book_id = orders.book_id) group by books.book_id, books.author, books.genre, books.price, books.publication_date