Looking for help on joining query - mysql

Question

im taking my very first programming (sql) class. I've got no tech background whatsoever and I'm having a little trouble getting the code down.
here is the what the database looks like.
BOOK
BOOK_CODE (UNIQUE)
BOOKTITLE
PUBLISHER_CODE
BOOKTYPE
PRICE
INVENTORY
BOOK_CODE (UNIQUE)
BRANCH_NUM (UNIQUE)
ON_HAND
The question is, list the books (titles) that are available in branches 3 and 4 (on both at the same time).
Im thinking i need to use the following tables: booktitle on the book table , bookcode on both tables ( book and inventory), and branch_num on inventory table.
also the answer can only show the book titles available on branches 3 and 4 (at the same time) and no other columns.
sorry if im making no sense. like i said im a n00b.

Answer 1

select distinct BOOKTITLE from BOOK a, INVENTORY b
where a.BOOK_CODE = b.BOOK_CODE and a.BOOK_CODE in
(select distinct p.BOOK_CODE from Inventory p, Inventory q where p.BOOK_CODE =
q.BOOK_CODE
and p.BRANCH_NUM = 3 AND q.BRANCH_NUM = 4);

Answer 2

SELECT BOOKTITLE FROM BOOK
WHERE BOOK_CODE IN (SELECT BOOK_CODE FROM INVENTORY WHERE BRANCH_NUM = 3 AND ON_HAND > 0)
AND BOOK_CODE IN (SELECT BOOK_CODE FROM INVENTORY WHERE BRANCH_NUM = 4 AND ON_HAND > 0);
OR
SELECT BOOKTITLE FROM
(
SELECT BOOK.BOOK_CODE, BOOKTITLE, COUNT(*) AS BRANCH_COUNT FROM BOOK
INNER JOIN INVENTORY ON BOOK.BOOK_CODE = INVENTORY.BOOK_CODE
AND INVENTORY.BRANCH_NUM IN (3, 4)
GROUP BY BOOK.BOOK_CODE, BOOKTITLE
) B
WHERE B.BRANCH_COUNT = 2;

Answer 3

Please try this:
SELECT BK.BOOKTITLE
FROM BOOK BK
INNER JOIN INVENTORY INV1
ON INV1.BOOK_CODE = BK.BOOK_CODE
INNER JOIN INVENTORY INV2
ON INV2.BOOK_CODE = BK.BOOK_CODE
WHERE INV1.BRANCH_NUM = 3
AND INV2.BRANCH_NUM = 4

Answer 4

Your question tells you the fields you need from the table, which gives you this as a starting point:
SELECT booktitle FROM book . . .
That alone will give you a list of every booktitle in the table book, but you want to filter it down to only those books in both branch_num 3 and 4.
When you JOIN two or more tables, you're looking to match rows based on some shared valued; book_code, in this case.
Since you want to see only those books in both tables, you'll want to use an INNER JOIN. (I'm assuming ON_HAND is an INT describing the number of copies in the branch.)
SELECT booktitle FROM book
INNER JOIN inventory USING (book_code)
WHERE inventory.onhand > 0
That query will return a list of every book that is available in any branch. Modifying the query, you can restrict it only to those books in branch_num 3:
SELECT booktitle FROM book
INNER JOIN inventory USING (book_code)
WHERE inventory.onhand > 0 AND inventory.branch_num = 3
...but you need books in both branches 3 and 4, so you'll need to do another join on the inventory table. But how can you distinguish the two? Use table aliases:
SELECT booktitle FROM book
INNER JOIN inventory AS inventory1 USING (book_code)
INNER JOIN inventory AS inventory2 USING (book_code)
WHERE inventory1.onhand > 0 AND inventory1.branch_num = 3 AND inventory2.onhand > 0 AND inventory2.branch_num = 4
That should give you what you're looking for.
For a much better explanation of joins, see MySQL Joins and A Visual Explanation of SQL Joins.
[Note: this problem could also be using subqueries, which you should try to do.]