在MySQL中形成子查詢

Question

使用子查詢，可以在哪個分支上找到「SQL指南」一書？列出分支名稱，分支位置和可用份數。不要明確測試書本代碼669X。讓MySQL爲你做好工作。在此處插入您的查詢和結果：

下面的底部代碼是否正確？我很困惑如何在子查詢中輸入此內容，這似乎更簡單

使用Henry_Books;

select branch_name,branch_location,inventory.on_hand 
from branch,inventory,book 
where branch.branch_num = inventory.branch_num 
and title = "A guide to SQl" ; 

Answer 1

無需子查詢：

select branch_name,branch_location, 
    COUNT(inventory.book_id) NumberOfCopies 
from branch 
inner join inventory on branch.branch_num = inventory.branch_num 
inner join book on inventory.book_id = book.book_id 
where book.title = "A guide to SQl" 
GROUP BY branch_name, branch_location; 

（我不知道是什麼，所以我猜測這將是book_id基地兩個表book和inventory之間的關係）

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如果需要使用子查詢來做到這一點，那麼你可以做到這一點，這是更復雜，根本不需要它：

select branch_name,branch_location, 
    (SELECT COUNT(inventory.book_id) 
    FROM inventory on 
    inner join book on inventory.book_id = book.book_id 
    WHERE branch.branch_num = inventory.branch_num 
    AND book.title = 'A guide to SQl' 
) AS NumberOfCopies 
from branch 

請注意，這是一個相關的子查詢。

Answer 2

簡單使用加入表之間，和你的情況分組

：

SELECT branch_name,branch_location,inventory.on_hand 
    FROM branch AS branch 
    LEFT JOIN inventory AS inventory ON (branch.branch_num = inventory.branch_num) 
    LEFT JOIN book AS book ON (branch.branch_num = book.branch_num) 
WHERE branch.branch_name = "A guide to SQl" 
    ORDER BY branch.branch_name 
    GROUP BY branch.branch_name 

---

如果你喜歡去與不推薦使用子查詢

SELECT branch_name,branch_location,on_hand AS (SELECT on_hand FROM inventory WHERE branch.branch_num = inventory.branch_num ) 
    FROM branch AS branch 
    LEFT JOIN book AS book ON (branch.branch_num = book.branch_num) 
WHERE branch.branch_name = "A guide to SQl" 
    ORDER BY branch.branch_name 
    GROUP BY branch.branch_name