Question

我写了以下查询：

select title, branch_name, no_of_copies 
from Book_Copies BC 
inner join Book B on BC.book_id = B.book_id 
inner join Library_Branch LB on BC.branch_id = LB.branch_id 
where title = 'Grapes of Wrath';

但我的问题是，这只显示了具有＆gt;的标题和分支名称。 0 no_of_copies。如何显示没有no_of_copies行的那些？（它等于0）。

以下是我目前获得的表格：

+-----------------+-------------+--------------+
| title           | branch_name | no_of_copies |
+-----------------+-------------+--------------+
| Grapes of Wrath | Central     |            5 |
| Grapes of Wrath | Allentown   |            2 |
+-----------------+-------------+--------------+

这就是我所需要的：

+-----------------+-------------+--------------+
| title           | branch_name | no_of_copies |
+-----------------+-------------+--------------+
| Grapes of Wrath | Central     |            5 |
| Grapes of Wrath | Allentown   |            2 |
| Grapes of Wrath | Sharpstown  |            0 |
+-----------------+-------------+--------------+

Answer 1

您需要使用outer join，而不是从book表开始。也许是这样的：

select title, branch_name, no_of_copies 
from Book B
    left join Book_Copies BC on BC.book_id = B.book_id 
    left join Library_Branch LB on BC.branch_id = LB.branch_id 
where title = 'Grapes of Wrath';

阅读您的评论，听起来您希望在cross join和book之间使用library_branch来获取图书和分支的所有组合（也称为cartesian product ）。然后outer join将这些结果发送到查找表：

select title, branch_name, coalesce(no_of_copies,0) copies
from Book B 
    cross join Library_Branch LB
    left join Book_Copies BC on BC.book_id = B.book_id 
                            and BC.branch_id = LB.branch_id 
where title = 'Grapes of Wrath';

显示没有值的查询中的项目

1 个答案: