FAQs on the exercise Many-to-Many Relationship Part 2
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What is the point of populating a many-to-many table like in this exercise? If ‘book’ and ‘author’ are populated, doesn’t ‘books_authors’ get populated?
The point is mapping the book to the author. An author can write many books, and a book can have many authors. The books to authors table will not get populated automatically if thats what you are asking.
Came here to say as someone that has been working in SQL for 20 years… I would suggest you do not write your code in this format as the exercise gives as an example. This is the old format that fell out of favor 15+ years ago. Use the explicit JOIN format.
SELECT column_one AS alias_one, column_two AS alias_two
FROM table_one, table_two, joined_table
WHERE table_one.primary_key = joined_table.foreign_key_one
AND table_two.primary_key = joined_table.foreign_key_two
Beyond looking old and outdated… the explicit JOIN method is much nicer for troubleshooting and performance tuning. You can quickly see which criteria is part of the join vs a filtering factor in the where.
#1
to clarify, are you saying that the outdated code is what you have in your comment and we should not use (which looks the same as the first option in the hint under q4 in the exercise) and that we SHOULD use the alternative code:
SELECT column_one AS alias_one, column_two AS alias_two
FROM table_one
INNER JOIN joined_table
ON table_one.primary_key = joined_table.foreign_key_one
INNER JOIN table_two
ON table_two.primary_key = joined_table.foreign_key_two
or should we use some other code altogether?
#2
Also, in the code you have in your comment, I’m confused because where q4 says “You should expect 3 …” the result excerpt has 3 fields, but in the code we are only selecting 2 columns… how is that right?
#3
Also in that same body of code, why are we selecting FROM ‘joined_table’ if we’re not displaying any columns from it?
Can someone please help figure out the proper way to write the following code?
Idea: book table paired with books_authors table - paired with author table
select book.title as book_title, author.name as author_name, book.description as book_description
from book, author
inner join books_authors
on book.isbn = books_authors.book_isbn
inner join author
on books_authors.author_email = author.email;
Just understand there are implicit and explicit ways of joining the tables. It may depend on the database you are working with and how old it is. In my Oracle RDBMS experience, I’ve only used implicit joins, where others using MySQL have only used explicit joins.
thanks for bringing that distinction to my attention!
I just fixed the syntax a bit and followed the format provided in the question 4 hint, but still error:
SELECT
book.title as book_title,
book.description as book_description,
author.name as author_name
FROM book
INNER JOIN books_authors
ON book.isbn = books_authors.book_isbn
INNER JOIN author
ON books_authors.author_email = author.email;
Thanks for including all the screenshots. Your SELECT statement was correct. I took the time to go thru that section of the course to see what the problem was.
Try ordering your column names exactly as specified in the question, book_title, author_name, and book_description, and your error message should disappear.
Hi thanks for the information, Im still confused.
I have the SELECT part, understood.
the FROM part as well all good so far
the WHERE is confusing me along with the AND part
Is the third table in play here? where we put the two primary keys into one table ( I didn’t want to use the word joined together).
This is my Code.
SELECT book.title AS book_title, author.name AS author_name, book.description AS book_description
FROM book, author, joined_table → (books_authors)
WHERE book.isbn = books_authors.author_isbn
AND author.email = books_authors.book_email;
then as I was writing this I realised that the third table that we created earlier in the tutorial came into play… thanks for helping me Ps the above code is wrong.
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thanks for helping me Ps the above code is wrong.