How to nest aggregation functions in JPQL

I'm working in Spring for a college project. I have to develop a system that allow users to create "meetings". I've mapped the relationship between User and Rendezvous(Meeting) like this:

User (1) <--- (0...*) Rendezvous

@Entity
@Access(AccessType.PROPERTY)
public class Rendezvous extends DomainEntity {

private User                        user;


@NotNull
@Valid
@ManyToOne(optional = false)
public User getUser() {
    return this.user;
}

public void setUser(final User user) {
    this.user = user;
}

}

I need to know the average and the standard deviation of rendezvouses created per each user, so I'm trying to create a query to do that.

I need to make an equivalent query to this one in mySQL but in JPQL

    select avg(rendezvouses) from (
       select user_id, count(*) as rendezvouses 
          from rendezvous group by user_id
    ) nestedQuery;