Question: How would you retrieve data on all the customers where no phone number is stored?

1. `SELECT * FROM customers WHERE PhoneNumber = NULL;`
2. `SELECT * FROM customers WHERE PhoneNumber IS NOT VALID;`
3. `SELECT * FROM customers WHERE PhoneNumber IS NULL;`
4. `SELECT * FROM customers WHERE PhoneNumber IS UNKNOWN;`

Answer: The correct answer of the above question is Option C:`SELECT * FROM customers WHERE PhoneNumber IS NULL;`