Drill 23: Explain each line — CASE expressions

SELECT title, minutes,
       CASE
         WHEN minutes >= 15 THEN 'Long'
         WHEN minutes >= 12 THEN 'Medium'
         ELSE 'Short'
       END AS length_band
FROM lessons
ORDER BY minutes DESC;

SELECT course_slug,
       COUNT(*) AS total,
       SUM(CASE WHEN minutes >= 15 THEN 1 ELSE 0 END) AS long_lessons
FROM lessons
GROUP BY course_slug
ORDER BY long_lessons DESC;