Let’s look at INNER JOIN
from the perspective of relations. Let’s recall one of our previous queries with INNER JOIN
we used to calculate the most popular book:
SELECT name, COUNT(DISTINCT(bu.user_id)) FROM books_users bu INNER JOIN books b ON bu.book_id = b.id GROUP BY 1 ORDER BY 2 DESC, name ASC
books_users and books has one-to-many relationship: One book has many books_users, in other words A record in books_users table belongs to one book. When we INNER JOIN
books to books_users we attach a row from books table to each row in books_users...
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