Aggregate Functions | Learn SQL Visual

Calculate One Value from Many Rows

Aggregate functions fold multiple rows into one value. COUNT returns a count, SUM a total, AVG an average, and MIN and MAX the smallest and largest values. COUNT(*) counts all rows, while COUNT(column) skips NULLs.

What is the difference between COUNT(*) and COUNT(city) for customers?

CHECK IT

customers6 rows

id	name	city
1	Alice	Tokyo
2	Bob	Osaka
3	Carol	Tokyo
4	Dave	Osaka
5	Erin	Nagoya
6	Frank	NULL

Aggregate functions calculate one value from multiple rows. customers has six rows with ids 1 through 6.