Aggregate Functions

Turning Many Rows Into One Summary

Aggregate functions collapse a column of values into a single number.

SELECT COUNT(*) FROM customers;
SELECT COUNT(city) FROM customers;      -- ignores NULLs
SELECT SUM(total), AVG(total) FROM orders;
SELECT MIN(price), MAX(price) FROM products;

The Core Five

• COUNT(*): number of rows; COUNT(col) ignores NULLs.
• SUM: total of numeric values.
• AVG: arithmetic mean.
• MIN / MAX: smallest and largest value.

Counting Distinct Values

SELECT COUNT(DISTINCT city) FROM customers;

Avoiding the Divide-by-NULL Trap

If no rows match, SUM, AVG, MIN and MAX return NULL, not zero. Wrap them when needed:

SELECT COALESCE(SUM(total), 0) FROM orders WHERE customer_id = 999;

Key Points

• Aggregates summarise a whole column into one value.
• COUNT(DISTINCT col) counts unique entries.
• Aggregates return NULL on empty input; COALESCE fixes display.
• They become truly powerful combined with GROUP BY.