Finding missing values with IS NULL

NULL means "no value here". Brightwell doesn't know some suppliers' email addresses, online orders weren't taken by an employee, and an order that hasn't shipped has no shipped date. All of these are NULL.

NULL isn't zero, and it isn't empty text. It means "unknown" or "doesn't apply".

To find rows where a column is NULL, use IS NULL:

SELECT name, country
FROM suppliers
WHERE contact_email IS NULL

And to find rows where it has a value, use IS NOT NULL:

SELECT id, order_date, shipped_date
FROM orders
WHERE shipped_date IS NOT NULL

NULL can mean something

Sometimes NULL is useful information rather than a gap. In employees, a NULL leave_date means the person still works at Brightwell. Always check what NULL means in a column before using it.