How to handle null and NULL value in sql

So I have been facing this weird situation in mysql where somehow null values are inserted in my table.
I’m talking about null value not NULL value.

I have attached image for better understating

as you can see name column have null and mobile_no have NULL

So after using this query

select Case when t1.name IS NULL then 'NA' when t1.name= 'NA' or 'null' or NULL then 'NA' else t1.name end as 'Name', 
Case when t1.mobile_no IS NULL then 'NA' when t1.mobile_no= 'NA' or 'null' or NULL then 'NA' else t1.mobile_noend as 'Mobile no' from student;

after this I’m getting this result

|Name|Mobile no|
----------------
|null|NA       |

but I want below result

|Name|Mobile no|
----------------
|NA  |NA       |

>Solution :

To compare a column with multiple values use col IN (x, y, z), not col = x OR y OR z.

You also can’t compare with NULL using = or IN, so that has to be a separate check.

select 
    Case 
        when t1.name IS NULL OR t1.name IN ('NA' or 'null') then 'NA' 
        else t1.name 
    end as 'Name'