NULL and aggregates

See how NULL affects aggregates.

Aggregate functions skip NULL values. Only COUNT(*) counts all rows.

SELECT COUNT(*), COUNT(email) FROM customers;

This query gives total customers and those with an email.

COUNT(*), COUNT(email) is selected from customers.

NULL and aggregates — details

COUNT(column) does not count NULLs. SUM, AVG, MIN, MAX also ignore NULLs.

That's why you decide whether NULL means zero or unknown in averages and totals, and use COALESCE when needed.

Find how many customers have an email with COUNT(email).

Show example solution

SELECT COUNT(email) FROM customers;

Add COUNT

SELECT ____(email) FROM customers;

Show answer

SELECT COUNT(email) FROM customers;