Final answer:
In SQL, the WHERE clause IS NULL is used to find rows with null values, while IS NOT NULL is used to find rows without null values in a specified column.
Step-by-step explanation:
The WHERE clause IS NULL and IS NOT NULL in SQL are used to test for null or not null values in a database. A null value indicates that the value is missing or unknown at the time of the record insertion. The IS NULL condition is used to retrieve rows where a column value is null. Conversely, the IS NOT NULL condition is used to retrieve rows where the column value is not null. For example, if we have a table of customers with a 'phone_number' column, we could use SELECT * FROM customers WHERE phone_number IS NULL; to find all customers without a phone number on record, or SELECT * FROM customers WHERE phone_number IS NOT NULL; to find those with a phone number.