SQL COUNT, SUM and AVG: totals and averages

COUNT(*) counts rows, SUM adds values, AVG averages them, and MIN and MAX find the extremes. ROUND(AVG(x), 2) keeps money readable.

Filter first with WHERE (for example only paid orders), then aggregate, or your total will include rows that should not count.

Worked example

How much revenue came from order lines, in total?

SELECT ROUND(SUM(quantity * unit_price), 2) AS revenue
FROM order_items;

Common mistakes

• Summing prices instead of quantity × price.
• Averaging a column that already contains averages.