Aggregate Functions

Aggregate functions compute a single value across many rows: COUNT() counts rows, SUM() totals a column, AVG() averages it, MIN()/MAX() find the smallest/largest value.

Example: an average score per track

SELECT track, AVG(score) AS avg_score
FROM students
GROUP BY track;

track         | avg_score
Frontend      | 90.0
Cybersecurity | 78.0
Backend       | 85.0

Without GROUP BY, AVG(score) alone would collapse the entire table into one single average across every student. Adding GROUP BY track changes that completely: it computes a separate average for each distinct track value, producing one row per group instead of one row total — the difference between "what's the average score overall" and "what's the average score, broken down by track," which is a genuinely different, much more useful question.