COUNT, SUM, AVG

Aggregate functions turn many rows into one number.

• COUNT(*) counts rows; SUM and AVG add up and average a column.
• MIN and MAX give the smallest and largest value.
• Combine with WHERE to summarise only some rows.

SELECT COUNT(*), MAX(marks), AVG(marks) FROM students;

5 | 91 | 80.4

Remember: Aggregates squash all the matching rows into one result.

Common mistake: Mixing an aggregate with a plain column without GROUP BY, like SELECT city, AVG(marks).