What is Null ? Null means no data or missing data or we don't have a data in a table . If you think Null means 0 ( zero ) than you are wrong .
There can be two condition for Null
1) when SQL is Null
2) when SQL is not Null
If in a table you write " Not Null " it means it is mandatory to enter a data in that column and if you don't write " Not Null " than it is not mandatory to fill that column . That means that field have automatically null value .

Example :

    create table student ( stu_id int not null , stu_name varchar ( 20 ) ) ; // Here id is mandatory

    create table student ( stu_id int , stu_name varchar ( 20 ) ) ; // Here id is not mandatory

          Employee_name            Employee_Age          Employee _Salary
            Mukesh                  23                         
            Ayush                   24                           200000
            Ishan                   20                          
            Pranav                  35                           700000
            Abhishek                26                         
            Ravi                    25                           300000
            David                   40                           800000

Query to select Null Fields :

   select  Employee_name , Employee_Age , Employee _Salary from student where Employee_Salary is null       ;

Output :

         Employee_name            Employee_Age          Employee _Salary
              Mukesh                  23     
              Ishan                   20  
              Abhishek                26      

2) When SQL is Not Null :
Output :

           Employee_name            Employee_Age          Employee _Salary
                Ayush                   24                       200000
                Pranav                  35                       700000
                Ravi                    25                       300000
                David                   40                       800000