我有這個經典的 sql 問題 How many copies of the book titled The Lost Tribe are owned by each library branch?
我通過獲取具有副本的分支解決了這個問題，但是有沒有辦法獲取所有分支并填充包含標題的 noOfCopies 并輸入 null 如果沒有？我試過左、內和右連接，但我無法弄清楚。
select l.branchName, r.noOfCopies
from (tbl_library_branch as l
join tbl_book_copies as r
on l.branchId = r.branchId)
right join tbl_book as bk
on r.bookId = bk.bookId
where title in ('The Lost Tribe');
uj5u.com熱心網友回復：
可能您在追求以下內容。從您需要所有行和外部連接到按標題過濾書籍的結果的表開始：
select b.branchName, coalesce(c.noOfCopies,0) noOfCopies
from tbl_library_branch b
left join (
select bc.branchId, bc.noOfCopies
from tbl_book b
join tbl_book_copies bc on bc.bookId=b.bookId
where b.title = 'The Lost Tribe'
)c on c.branchId = b.branchId;
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