Database Design – Designing a Simple Library Database

database-designms access

I am currently developing a simple book library system in ms access 2007 and need some help with the database design. I want to register books, borrowers and loans, so I assume that my database could consist of these three entities.

Borrower
Loan
Book

The relationships between the tables are as so:

Borrower has a 1 to many relationship with Loan.

Loan has a 1 to many relationship with Book.

The problem with this approach is that you can not create a row in the Book table without assigning a foreign key value; thus referencing the foreign key loanId of Book table with the related primary key in the Loan table. It breaks the referential integrity and it's not possible to create a book without relating it to a loaner.

So I think I need a join table between Book and Loan. This table should consist of the 2 primary keys from Book and Loan (a composite key). So there is a 1 to many relationship between Book and BookLoan and a 1 to many between Loan and BookLoan.

Is my way of thinking correct ? I can't figure out why I need the join/junction table between loan and book. Is there a more simple solution to my problem?

Thanks in advance 🙂

EDIT: I have added a E/R-diagram which has been corrected in respect to the answer, I have received.

Best Answer

I believe you are assigning the Foreign key on Book to Loan. It should be just the opposite and be on Loan to Book. The Book doesn't need a comparative loan but every loan needs a comparative book.

Both of your foreign keys should exist on Loan.

Related Solutions

Many to Many and Weak Entities

I think you can, using a "diamond" relationship diagram:

diagram

CREATE TABLE Artist
( artistID INT NOT NULL
, name VARCHAR(100) NOT NULL
, PRIMARY KEY (artistID)
) ;

CREATE TABLE Album
( artistID INT NOT NULL
, albumID INT NOT NULL
, title VARCHAR(100) NOT NULL
, PRIMARY KEY (artistID, albumID)
, FOREIGN KEY (artistID)
    REFERENCES Artist (artistID)
) ;

CREATE TABLE Track
( artistID INT NOT NULL
, trackID INT NOT NULL
, title VARCHAR(100) NOT NULL
, PRIMARY KEY (artistID, trackID)
, FOREIGN KEY (artistID)
    REFERENCES Artist (artistID)
) ;

CREATE TABLE AlbumTrack
( artistID INT NOT NULL
, albumID INT NOT NULL
, trackID INT NOT NULL
, trackNo INT NOT NULL
, PRIMARY KEY (albumID, trackNo)
, FOREIGN KEY (artistID, albumID)
    REFERENCES Album (artistID, albumID)
, FOREIGN KEY (artistID, trackID)
    REFERENCES Track (artistID, trackID)
, UNIQUE (trackID, albumID)               -- this Unique constraint should be added
                                          -- if no track is allowed twice in an album
) ;

Database Design – Relate Two Tables to a Third Table

Here is a suggestion so you can enforce the constraints you want declaratively. (I've simplified the table names a bit, removed the bridge_ prefix.)

We remove footnote_num from:

Table:      a_ref         -- was named:  bridge_a_reference
     a_id, 
     ref_id
Primary key: 
     (a_id, ref_id)
Foreign keys:
     a_id  -> a
     ref_id -> reference

We add this table - which will basically store only those rows from a_ref with footnote, those you want to add children into the b_ref:

Table:      a_ref_with_footnote 
    a_id, 
    ref_id,
    footnote_num
Primary key: 
    (a_id, ref_id)
Unique key:
    (a_id, footnote_num)
Foreign keys: 
    (a_id, ref_id)  -> a_ref

And finally the 3rd table stays as in your design except the foreign keys which now reference the intermediate table (a_ref_with_footnote):

 Table:     b_ref         -- was named: bridge_b_reference:
     a_id, 
     b_id, 
     ref_id,
 Primary key: 
     (b_id, ref_id)
 Foreign keys: 
     (a_id, b_id) -> b
     (a_id, ref_id) -> a_ref_with_footnote

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