LEFT JOIN mixed with grouping and counting is the basis for histogram analysis. Number of readers per books is just one of the examples
LEFT JOIN keeps all records from the left table we’ll have all of them in our distribution, even if they have no associated records in the right table.
Let’s finish our example with counting readers per book:
SELECT * FROM books b LEFT JOIN books_users u ON u.book_id = b.id
Unlike in a joined table with
INNER JOIN some of the right table records are just
NULL-s If we run our typical grouping and...