When we don't define any data for a column then the column holds the NULL value. In MySQL, NULL values are used to represent data that is missing.

NULL Values

If you have created a table with optional columns then you can insert and update records for that table without inserting/updating data into optional columns in that case those columns will be saved with NULL value.

Example:

We have a table "user" as below:

user

id  first_name            last_name       country
.......................................................
1   John                     Simp         Canada
2   Chris                    Hely         Korea
3   Joy                      Roy          
4   Jenny                    Mill     

Suppose that the "country" column into the "user" table is optional, that means if we insert a new record with no value in "country" column then NULL value will be saved in "country" column.

Now the main question is how can we test column for NULL values? We can not test for NULL values with comparison operators (=,<,<>). For that we'll have to use IS NULL and IS NOT NULL operators provided by MySQL.

IS NULL

Suppose we want to select the records with NULL values only in the "country" column, for that we'll use IS NULL operator as below:

SELECT first_name,last_name,country FROM user
WHERE country IS NULL;

Result:

first_name      last_name      country

........................................................

Joy             Roy            
Jenny           Mill   

IS NOT NULL

Suppose we want to select the records with no NULL values in the "country" column, for that we'll use IS NOT NULL operator as below:

SELECT first_name,last_name,country FROM user
WHERE country IS NOT NULL;

Result:

first_name      last_name      country
..............................................
John            Simp           Canada
Chris           Hely           Korea

Hope this will help you :)