Drill 21: Beginner mistake — NULLs and COALESCE

SELECT c.title, l.title AS lesson
FROM courses c
LEFT JOIN lessons l
  ON l.course_slug = c.slug AND l.minutes > 15
ORDER BY c.title, l.title;

SELECT c.title,
       COALESCE(l.title, 'no long lesson') AS longest
FROM courses c
LEFT JOIN lessons l
  ON l.course_slug = c.slug AND l.minutes > 15
ORDER BY c.title;