How to add column values in mysql

Sum is a aggregate function. You dont need to use it. This is the simple query -

select *,(maths + chemistry + physics ) AS total FROM `student`

Tip: If one of the fields has the possibility to be NULL, then use COALESCE to default these to 0, otherwise total will result in NULL.

SELECT *, (
    COALESCE(maths, 0) +
    COALESCE(chemistry, 0) +
    COALESCE(physics, 0)
) AS total 
FROM `student`