Counting and summarizing

One number from many rows

Aggregate functions reduce a whole column to a single value:

- COUNT(*) — number of rows - SUM(price), AVG(price) — total / average - MIN(price), MAX(price) — smallest / largest

SELECT COUNT(*) AS books, AVG(price) AS avg_price
FROM books;

Aggregates ignore NULL (except COUNT(*)). They also respect WHERE, so you can summarize a subset.