Question

这是我简单的SQL问题......

我有两张桌子：

图书

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

订单

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

我想创建一个返回：

的查询

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

换句话说，返回Books表中 ALL 行的每一列，以及一个名为“number_of_orders”的计算列，该列计算每本书在Orders表中显示的次数。（如果订单表中没有出现图书，则应一书列在结果集中，但“number_of_orders”应为零。

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

这几乎正确，但并不完全，因为即使书籍从未在Orders表中列出，“number_of_orders”也将为1。此外，由于我缺乏SQL知识，我确信这个查询效率非常低。

编写此查询的正确方法是什么？（对于它的价值，这需要适用于MySQL，因此我不能使用任何其他特定于供应商的功能。）

提前致谢！

Answer 1

您的查询几乎是正确的，这是正确的方法（并且效率最高）

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(*)可能在计数中包含NULL值，因为它会计算所有行，而COUNT(orders.book_id)则不会，因为它会忽略给定字段中的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

你算错了。您想要计算非null book_id。

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

Answer 5

select author.aname,count(book.author_id) as "number_of_books"
from author 
left join book 
on(author.author_id=book.author_id)
GROUP BY author.aname;

如何计算表中每个外键ID的实例数？

5 个答案: