Question

I’m designing a library application for my school. We have a 2-3 main libraries and some departments have their own libraries. I wanted to know whether these queries seem correct ?

Libraries 
ID Name
1  Walter
2  Wally
3  Maths dept library

Books
ID Book ID  BookName
1   1       Fundamentals of calculus
2   1       Mechanics 
3   2       Fundamentals of calculus
4   2       Biology

Lib_Book_LookUp
ID  libId bookId
1    1     1
2    1     2
3    2     1
4    2     2

The two questions I want to solve are:

1. Find how many copies of a particular book exist
2. Find out which libraries have a particular book

Here’s what I have:

Select count(Book.bookId) where book.name = "Fundamentals of calculus";

Select count(libId) from Lib_Book_lookUp, Book where Book.BookId = Lib_Book_lookUp.bookId groupBy(libId)

My questions regarding these queries:

1. Does it seem unnecessary to have a separate lookup table and merge the libId into Books table and have it as a foreign key on the book table ?
2. Do the queries seem correct ?

Answer 1

Your first query is missing a FROM clause. You could also do COUNT(*), but it doesn’t particularly matter in this case:

 SELECT count(Book.bookId) FROM Books WHERE BookName = "Fundamentals of calculus";

To find out which libraries have a particular book, you don’t need a COUNT() aggregate. Instead you need a WHERE clause with your join. The COUNT() would tell you how many libraries have a book, but not which libraries. For that you want a query which returns Libraries.Name.

SELECT
  /* Return library names */
  Libraries.Name
FROM 
  Libraries 
  /* Join through your lookup table to match a book name to a library name */
  JOIN Lib_Book_LookUp ON Libraries.ID = Lib_Book_LookUp.libId
  JOIN Books ON Books.ID = Lib_Book_LookUp.bookId
/* Which book to search for */
WHERE Books.BookName = 'Fundamentals of calculus'

The lookup table is appropriate because it allows you to normalize books down to a single record in the Books table if it exists in multiple libraries. As you have it, there is no real reason to have multiple copies in the Books table, and no reason for the Book ID column. In fact, the Book ID column as it is is quite misleading. There are books with two different titles having the same id 1.

The Books table really ought to look like the following, with one record per book title (assuming title as the authority, forgetting about real authoritative things like ISBN)

Books
ID BookName
1  Fundamentals of calculus
2  Mechanics 
3  Biology

If you have multiple copies of each book possible at each library, you may consider normalizing the books per copy into a table that identifies them by library barcode. You would then match those ids to libraries as holdings:

Books (defines bibliographic details)
ID BookName
1  Fundamentals of calculus
2  Mechanics 
3  Biology

Book_Copies (Matches Books.ID to barcode, barcode is Primary Key)
BookId Barcode
1      1234567
1      1234568
2      8654321
2      8654322

Lib_Book_LookUp (matches book copies to libraries, allowing multiple copies by barcode)
ID  libId bookBarcode
1    1     1234567
2    1     1234568
3    2     8654321
4    2     8654322

To query, for example, the number of copies per book per library, you would use:

SELECT 
  Libraries.Name,
  Books.BookName,
  COUNT(*)
FROM 
  Libraries
  JOIN Lib_Book_LookUp ON Libraries.ID = Lib_Book_LookUp.libId
  JOIN Book_Copies ON Lib_Book_Lookup.bookBarcode = Book_Copies.Barcode
  JOIN Books ON Book_Copies.BookId = Books.ID
GROUP BY 
  Libraries.Name, 
  Books.BookName