Update data in database using SQL

Thats a bad database design never store the data as comma separated string, this will make things messy in future.

You should think of normalizing the table something as for the student your table should look like

- id primary key auto_incremented
- name 
- other columns related to student

Then another table as student_qualification

- id primary key auto_incremented
- id_student ( id from student table)
- qualification

So for each student you can add as many qualification as possible to this table and can easily do add/edit/delete data

You can later easily retrieve data using simple joining the table.

first u have to select your existing value of Qualification column that u want to update

Using

select qualification from  tb_name where id = 1

Using above query u will get your qualification column value

suppose in

$qulification

Now update that row using

update set tb_name set qualification = '".$qualification."."your new value" where id = 1

May you can try this

 update employee set qualification = qualification || ',MCA' where id = 1

the above will work in oracle

EDIT:

Then you can have the case statement with it

   update employee set qualification = case when qualification is null then
                                     'MCA' else qualification || ',MCA' end where id = 1

You can test for NULL in the SET clause, and use concatenation to format the string appropriately.

update student
set qualification = concat(if (qualification is not null
                               , concat( qualification, ',')
                               , '' )
                           , 'MBA')
where id = 1;

Here is a working SQL Fiddle (which also demonstrates behaviour with a NULL qualification).

I agree with @Abhik that this is a bad design for this specific data, and normalization is the better approach for the use case you provide However, there are other use cases where doing this sort of update would be perfectly valid so the question is worthy of a proper answer..