顯示項目沒有價值

Question

我寫了下面的查詢：

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'; 

但我的問題是，這種只顯示標題和具有> 0 no_of_copies分支名稱。如何顯示沒有no_of_copies行的行？ （它將等於0）。

下面是表我目前得到：

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

這就是我需要的是：

+-----------------+-------------+--------------+ 
| 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而不是從book表開始。也許是這樣的：

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'; 

---

閱讀您的意見，這聽起來像你想使用library_branch一個cross join之間book得到的書籍和分支機構所有組合（或者稱爲cartesian product）。然後outer join這些結果與查找表：

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';