MySQL NULL VALUE
NULL is defined as no value in MySQL
If your record in the table has no value for a particular field you can insert
NULL value for that
NULL value for the salary field of the employee
INSERT INTO employee VALUES ("John Lee",42,null)
It is not possible to test for NULL values with comparison operators, such as =, <, or <>.
We will have to use the
IS NULL and
IS NOT NULL operators instead.
SELECT name FROM employees WHERE salary IS NOT NULL