Using IS NULL and IS NOT NULL Conditions

The IS NULL condition is satisfied if the specified column contains a NULL value, or if the specified expression evaluates to NULL.

If you use the IS NOT NULL predicate, the condition is satisfied when the column contains a value that is not NULL, or when the expression does not evaluate to NULL. The following example selects the order numbers and customer numbers for which the order has not been paid:
SELECT order_num, customer_num FROM orders
   WHERE paid_date IS NULL;

For a complete description of the IS NULL and IS NOT NULL operators, see the IS NULL and IS NOT NULL Conditions.