MYSQL查询图书数量

Question

  BOOK_COPIES (Book_ISBN [pk,fk], Branch_ID [pk,fk], Num_Copies)
  BOOK_LOANS (Book_ISBN [pk,fk], Branch_ID [pk,fk],Card_Num [pk,fk], Date_Out, Date_Due)
  LIBRARY_BANCH (Branch_ID [pk], Branch_Name, Address)

   Q. For each branch list the branch ID and the number of books they have. This is the     
      total number of books including multiple copies.

解决方案:从图书馆分支lb中选择lb.branchid，bookcopies bc其中bc.branchid=b.branchid group by lb.branchid;

但是我发现很难获得他们拥有的书籍数量？

Answer 1

试试这个，它应该有效:

SELECT Branch_ID, SUM(Num_Copies) totcopies
FROM BOOK_COPIES
GROUP BY Branch_ID