Question

i have a database where i cannot figure out how to fetch the book data. i’ll be explaining how i did it before and how i plan to do it now.

to make it simple, i’ll take the “books” example since it’s the easiest to show the relationship. before, i had this structure which i found heavy on the DB:

Books:
book_id
title 
author_name
genre_name

it was easy to query individual books, all books, books by author and books by genre. the problem here was when i need to get the list of authors or genres. i had to select ALL books, detect possible duplicates, identicals, uniques etc. before handing them over to the browser. i already felt the lag in processing.

so what i plan to do now is this: i have 3 tables, one for books, another for authors and another for genres.

Books:       Authors:      Genre: 
book_id      author_id     genre_id
title        author_name   genre_name
author_id
genre_id

this way, i can tag the books by author_id and genre_id as well as easily get all authors and genres more quickly without affecting the “get by author” and “get by genre”. Also, i can easily change the book author and/or genre by altering author_id and genre_id.

but if i select a book, how do i do it when the normal select * from Books where book_id=something returns only the ids of genres and authors? how do i do it to return author names and genre names? subquery? a separate query? or is there actually a one-liner query that i don’t know about?

Answer 1

You need to use a INNER JOIN so you could do something like this

SELECT b.book_id, b.title, a.author_name, g.genre_name FROM Books 
INNER JOIN Authors a ON b.author_id=a.author_id 
INNER JOIN Genre g ON b.genre_id=g.genre_id
WHERE book_id=12

As an alternative if you did not want to create separate tables (which is good design by the way) you could use a DISTINCT query

 SELECT DISTINCT author_name FROM Books;