Null values is field that has no value. If a field in a table is optional, we can insert a new record or update record without adding a value to this field.
<?php
SELECT column_names
FROM table_name
WHERE column_name IS NULL;
?><?php
SELECT CustomerName, ContactNo, Address
FROM Customers
WHERE ContactNo IS NULL;
?><?php
SELECT column_names
FROM table_name
WHERE column_name IS NOT NULL;
?><?php
SELECT CustomerName, ContactNo, Address
FROM Customers
WHERE ContactNo IS NOT NULL;
?>