NULL in a table represents that the field has no value. NULL is different from zero and space. While inserting and updating the rows, there might be an optional column in table. If the optional column value not specified in the INSERT statement, the column gets updated with NULL value by default.

To verify the column value is NULL or NOT, we can use "IS NULL" or "IS NOT NULL". We should make a note that the NULL value can't compare with operators like =, >, <.

SELECT column_names
FROM table_name
WHERE column_name IS NULL; 

The above query fetches the rows that column_name having NULL values.

SELECT column_names
FROM table_name
WHERE column_name IS NOT NULL; 

The above query fetches the rows that column_name not having NULL values.

Let us consider below table(s) as an example table(s) to frame the SQL query for getting the desired results.

Scenario – Fetch rows which are having NULL values.

Requirement - Fetch employee details who has no manager assigned from employee_details table. The query was as follows -

 SELECT * FROM employee_details WHERE manager_id IS NULL; 

By executing above query, we can get results as shown below –

Scenario – Fetch rows which are not having NULL values.

Requirement - Fetch employee details who has a manager assigned from employee_details table. The query was as follows -

SELECT * FROM employee_details WHERE manager_id IS NOT NULL; 

By executing above query, we can get results as shown below -

Scenario – Update rows which are having NULL values in a column.

Requirement – Update manager details for the employee with id 001 in employee_details table. The query was as follows -

UPDATE employee_details
SET manager_id = 'Employee0' 
WHERE manager_id IS NULL AND emp_id = 001;     

By executing above query, the corresponding row gets updated with manager0 as manager_id like below -

Scenario – Deleting the rows which are having NULL values in a column.

Requirement – delete employee who are having manager_id as NULL. The query was as follows -

 DELETE FROM employee_details WHERE manager_id IS NULL; 

By executing above query, rows having manager_id gets deleted and the result table as shown below -