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Lesson 18 +10 XP

Aggregate Functions

Aggregate Functions

Aggregate functions compute a single value from many rows.

The most common ones

FunctionReturns
COUNT()how many rows
SUM()total of a numeric column
AVG()average of a numeric column
MIN()smallest value
MAX()largest value

Examples

SELECT COUNT(CustomerID) FROM Customers;
SELECT SUM(Price) FROM Products;
SELECT AVG(Price) FROM Products;
SELECT MIN(Price) FROM Products;
SELECT MAX(Price) FROM Products;

COUNT vs COUNT(*)

  • COUNT(column) counts rows where that column is not NULL.
  • COUNT(*) counts all rows.

They ignore NULL (except COUNT(*))

  • SUM, AVG, MIN, MAX ignore NULL values.
  • COUNT(column) ignores NULLs too.

Combine with WHERE

SELECT COUNT(*) FROM Orders
WHERE CustomerID = 1;

Counts only the rows that match the condition.

TL;DR

  • Aggregates turn many rows into one value.
  • COUNT, SUM, AVG, MIN, MAX.
  • They ignore NULLs, except COUNT(*).
  • WHERE filters before aggregating.