A GROUP BY question

I am trying to write, what is for me, a fairly complex query. It uses JOINS,
and also GROUP BY. I have this working with the exception of adding the
GROUP BY clause. 

Is there some reason I cannot add a GROUP BY function to a JOIN?

Here is what I have:


CREATE OR REPLACE view tasks_view as 
select 
    project.proj_no ,
    employee.first_name ,
    employee.last_name ,
    employee.id ,
    task_instance.hours , 
    work_type.type,
    work_type.descrip,
    rate.rate,
    employee.hourly_rate ,
    rate.rate * task_instance.hours as result ,
    SUM(rate.rate * task_instance.hours) 
    ^^^^^^^^^^^^^^
from 
    task_instance
GROUP BY 
^^^^^^^^^^^^^^^^^^
    project.project_key 
^^^^^^^^^^^^^^^^^^^^^^
join rate on 
    rate.employee_key = task_instance.employee_key
    AND
    rate.work_type_key = task_instance.work_type_key
inner join employee on
    rate.employee_key = employee.employee_key
inner join work_type on
    rate.work_type_key = work_type.work_type_key
inner join project on
    project.project_key = task_instance.project_key
ORDER BY 
    project.proj_no ,
    employee.id
    ;

The underlined lines are what I added, and I get a syntax error pointing to
the join. This works fine without the added lines.

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