在MySQL

Question

使用子查询，可以在哪个分支找到“SQL指南”？列出分支名称，分支位置和可用副本数。不要明确测试书籍代码669X。让MySQL为您完成工作。在此处插入您的查询和结果：

下面的底部代码是否正确？我对如何在子查询中输入它感到困惑，这似乎更简单

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

不需要子查询：

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;

（我不知道两个表book和inventory之间的关系是什么，所以我猜它将基于book_id）

如果需要使用子查询，那么你可以这样做，这更复杂，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

请注意，这是一个相关的子查询。

Answer 2

简单使用表之间的连接和分组

在你的情况下：

    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 

如果您想使用不推荐的Sub Query

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