SQL & Databases·Relating and summarising

Aggregation and GROUP BY

Summarise thousands of rows into the handful of numbers that matter.

Beginner 14 min read Lesson 8 of 480 Challenge included

What you will be able to do

Use COUNT, SUM, AVG
Group and filter groups
Understand HAVING vs WHERE

Aggregate functions collapse rows into values. GROUP BY defines the buckets; HAVING filters the buckets after aggregation.

summary.sqlSQL

SELECT course_slug,
       COUNT(*) AS total,
       ROUND(AVG(minutes), 1) AS avg_minutes,
       SUM(minutes) AS total_minutes
FROM lessons
WHERE level = 'Beginner'
GROUP BY course_slug
HAVING COUNT(*) >= 3
ORDER BY avg_minutes DESC;

Your turn

Average lesson length

Hard

Return each course_slug and its average lesson minutes, only for courses with at least 3 lessons.

SQL · starter

SELECT course_slug, AVG(minutes)
FROM lessons

Check your understanding before moving on.

Knowledge checkQuestion 1

Which clause filters aggregated groups?

Select an answer to continue

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Next: NULLs and COALESCE

Lesson 8 of 480 · 0% of course