Lesson 16 — LEFT JOIN: keep rows without a match

LEFT JOIN: keep rows that have no match

An INNER JOIN drops rows with no partner: a customer with no orders vanishes. A LEFT JOIN keeps every row of the left table; where no match exists, the right table's columns are NULL (SQL's "no value", shown as None in Python). COALESCE(x, 0) replaces NULL with 0.

SELECT a.name, p.title
FROM authors a LEFT JOIN posts p ON p.author_id = a.id

Why it matters: "customers who never ordered" and "products never sold" are only visible with a LEFT JOIN — often the most valuable rows in a report.