I find that Hibernate supports casecade query using HQL.

for instance, we have two classes:
1) School, which has properties: Long id and String name
2) User, which has properties: Long id, String name and School school

we can query by executing "select name,school.name from User" to get a user's name and school name.

but we can't execute "update User set name='xxx' , school.name='yyy' where id=zzz".

I use update in HQL because it can only update the non-null properties.

Is there a way to do this?

Why don't you do it like this?
load User
User setName ("xxx")
User setSchool(new School("yyy"))
save User

It can work, but executing "Load User" will cost unnecessary time.

Because in that way, hibernate will update the null value and clear the original data, say we have property "String address" for School, I want to remain address as it is and only update user.name and school.name.

HQL update can only update the fields we SET and keep others no changed. Thanks for reply.