Update just some of the fields

[Sayyed Mohammad Emami Razavi]

In MySQL when i execute full update query, all fields will be updated or
no? It means, which fields that have not been changed will not be updated?!

e.g.
current values for table named 'test', i want to update just desc:
id, name, desc
1, test1, test2

update test set name='test1', desc='test10' where id=1;

I use one framework that still does not implement update just changed
fields, i know that i can use this:

update test set desc='test10' where id=1;

any ideas?!