Creating IF statement that if "Variable" != NULL then WHERE clause

Question

I want to have a query that will let me select ALL from the table if the variable is null, and my only option here is PURE MYSQL here is my CODE

SELECT *
FROM tblPersonalData
if(VARIABLE!=null,WHERE    Studno=VARIABLE},'all')

Answer 1

It's simple OR operation:

select
*
from tblPersonalData
where variable is null
or studNo = variable;

Answer 2

You can emulate this behavior with the logical or operator. Note, however, that null is not a value, and you should test it explicitly with the is or is not operators, not = or !=:

SELECT *
FROM   tblPersonalData
WHERE  variable IS NULL OR sutdno = variable