Summarising with aggregate functions

Sometimes you do not want the rows themselves, but a summary of them. Aggregate functions turn many rows into a single value:

• COUNT(*) — how many rows
• SUM(col) — the total
• AVG(col) — the average
• MIN(col) / MAX(col) — the smallest / largest

SELECT COUNT(*) FROM student;

Naming and rounding the result

Give the result a clear heading with AS. Averages often have long decimals, so wrap them in ROUND(value, 2) to keep 2 decimal places:

SELECT COUNT(*) AS n, ROUND(AVG(score), 2) AS avg_score
FROM student;

You can combine several aggregates in one query.

Common mistakes

• COUNT(*) counts rows; AVG(col) averages a column.
• Round an average with ROUND(x, 2).