SQL IS NULL and NOT NULL

In SQL, the keywords IS NULL and IS NOT NULL are used to test whether a value in a column is NULL or not. NULL is a special value in SQL that represents an unknown or missing value.

The syntax for the IS NULL operator is as follows:

SELECT column_name(s)
FROM table_name
WHERE column_name IS NULL;

For example, suppose we have a table named "employees" with a column named "email_address". We can use the IS NULL operator to find all employees who do not have an email address, like this:

SELECT *
FROM employees
WHERE email_address IS NULL;

This query will return all rows from the "employees" table where the "email_address" column is NULL.

The syntax for the IS NOT NULL operator is as follows:

SELECT column_name(s)
FROM table_name
WHERE column_name IS NOT NULL;

For example, suppose we want to find all employees who have an email address. We can use the IS NOT NULL operator, like this:

SELECT *
FROM employees
WHERE email_address IS NOT NULL;

This query will return all rows from the "employees" table where the "email_address" column is not NULL.

Note that the comparison operators (= or <>) cannot be used to test for NULL values, because NULL is not a value but a condition representing an unknown value. To test for NULL values, we must use the IS NULL or IS NOT NULL operator.