mysql - 用于查找每本书至少有 2 份副本的分支的分配 SQL

Question

   Q. List the branch id and name of all branches that have atleast 2 copies of every  
       book.

表格:

    BOOK (Book_ISBN [pk], Title, Publisher_Name [fk])
    BOOK_COPIES (Book_ISBN [pk,fk], Branch_ID [pk,fk], Num_Copies)
    LIBRARY_BANCH (Branch_ID [pk], Branch_Name, Address)

但是，我尝试做这个问题，但问题是检查 2 个副本。

Answer 1

您可能会在 relational division 上找到这篇文章有用。

SELECT L.Branch_ID,
       L.Branch_Name
FROM   LIBRARY_BRANCH L
WHERE  NOT EXISTS (SELECT *
                   FROM   BOOK B
                   WHERE  NOT EXISTS(SELECT *
                                     FROM   BOOK_COPIES BC
                                     WHERE  BC.Book_ISBN = B.Book_ISBN
                                            AND Num_Copies >= 2
                                            AND L.Branch_ID = BC.Branch_ID))