SQL & Databases·Relating and summarising

Joins

Combine tables through keys, and know when each join type is appropriate.

Beginner 16 min read Lesson 7 of 480 Challenge included

What you will be able to do

Identify primary and foreign keys
Write inner and left joins
Avoid fan-out mistakes

Tables stay small and focused. Joins reconnect them through shared keys: a lesson belongs to a course, so lessons.course_slug references courses.slug.

joins.sqlSQL

SELECT c.title AS course, COUNT(l.id) AS lessons
FROM courses c
LEFT JOIN lessons l ON l.course_slug = c.slug
GROUP BY c.title
ORDER BY lessons DESC;

Join	Keeps
INNER JOIN	Only rows that match on both sides.
LEFT JOIN	All left rows, plus matches on the right.
RIGHT JOIN	All right rows, plus matches on the left.
FULL JOIN	All rows from both sides where supported.

Your turn

Course with lesson counts

Hard

List every course title with its number of lessons, including courses with zero.

SQL · starter

SELECT c.title, COUNT(l.id) AS lessons
FROM courses c

Check your understanding before moving on.

Knowledge checkQuestion 1

Which join keeps every row from the left table even without a match?

Select an answer to continue

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Next: Aggregation and GROUP BY

Lesson 7 of 480 · 0% of course