ruby on rails group by with null values problem

I have an hour table in witch I store user time tracking information, the table consists from the following cells

project_id
task_id (optional can be null)
worker_id
reported_date
working_hours

each worker enters several records per day so generally the table is looking like this

id project_id  worker_id task_id   reported_date  working hours;    
== =========== ========= ========= =============  ==============
1  1           1         1         10/10/2011      4                 
2  1           1         1         10/10/2011     14                 
3  1           1                   10/10/2011      4                 
4  1           1                   10/10/2011     14                 

the task_id is not a must field so there can be times when the user is not selecting it and their task_id cell is empty

now i need to display the data by using group by clause

so the result will be something like this:

project_id worker_id task_id   working hours
========== ========= ========= ==============
1          1         1         18                 
1          1                   18            

I did the following group by condition:

@group_hours = Hour.group('project_id,worker_id,task_id)').
      select('project_id, task_id ,worker_id,sum(working_hours) as开发者_C百科 
         working_hours_sum')

My view looks like this

<% @group_hours.each do |b| %>
     <tr>
       <td><%= b.project.name if b.project %></td>
        <td><%= b.worker.First_name if b.worker %></td>
        <td><%= b.task.name if b.task %></td>
        <td class="center"><%= b.working_hours_sum %></td>
       <td></td>
     </tr>
<% end %>

This it is working but only if the task_id is not null when task id is null it present all the records without grouping them like this

project_id  worker_id task_id   working hours
=========== ========= ========= ==============
1           1         1         18                 
1           1                    4            
1           1                   14    

I will appreciate any kind of solution to this problem

---

Assuming you are using mysql you can use the COALESCE function to work with nulls, by changing the null value to something else

e.g.

select COALESCE(colname,0) from table where COALESCE(colname,0) > 1;

IFNULL() is another option