Include null/0 rows in a group by - sql

Question

I want to query a database for users and the the amount of time they spent on each Activity Category. These categories are stored in the table ActivityCategory (int Id, varchar Name). There's only 8 of them and I want to see all of them, even when nobody spent time on a specific category.
I have the following query:
select u.NoEmploye, u.FirstName, u.LastName, Total=sum(h.NbHeures), ac.Name
from Users u
join Semaines s on u.EntityGuid=s.UserGuid
join HeuresProjets hp on s.Id=hp.WeekId
join Heures h on hp.HPId=h.HpGuid
join ActivityCodes code on h.Code=code.ActivityId
join ActivityCategory ac on code.Categorie=ac.Id
group by u.NoEmploye, u.FirstName, u.LastName, ac.Name
order by u.NoEmploye
It works fine but it doesn't return unused ActivityCategories. I tried every combination of full/left/right/inner/outer/you-name-it joins. The best I could get is completely null rows when a category is used by nobody and a null ac.Name for categories a specific user doesn't use but others do. I suspect the group by [...]ac.Name part is what's "eating" the unused categories. What am I doing wrong? Should I write a select query the a second one to group the results? I'll have a dozen more similar queries to write so I'd like to understand instead of just having a fixed query with no explanation.
EDIT
Lamak's second query works so far but it has the same problem when I add a where clause.
EDIT2 ypercube' edit works perfectly
Now let's see if I understand the query correctly.
I coalesce the Sum with 0 to have a proprer value when the result would be null. I start the selection from ActivityCategory to make sure I have all of them, even the unused ones, which I cross join with users to have every combination of ActivityCategory and Users. I left join Activitycodes to only get relevant rows and then I inner join my other tables to get to Semaine. My condition are applied to the Semaine table's join because I want to filter my data before the cross join. Finally, I group my data by Users and ActivityCategory to have the sum works.

Answer 1

This should do:
SELECT u.NoEmploye, u.FirstName, u.LastName, Total=sum(h.NbHeures), ac.Name
FROM ActivityCategory ac
LEFT JOIN ActivityCodes code
ON code.Categorie=ac.Id
LEFT JOIN Heures h
ON h.Code=code.ActivityId
LEFT JOIN HeuresProjets hp
ON hp.HPId=h.HpGuid
LEFT JOIN Semaines s
ON s.Id=hp.WeekId
LEFT JOIN Users u
ON u.EntityGuid=s.UserGuid
GROUP BY u.NoEmploye, u.FirstName, u.LastName, ac.Name
ORDER BY u.NoEmploye
Basically, if you want all Categories, you need to use that table as the first table on your FROM and do LEFT JOINs to that table.
UPDATE
If you want every category for every user on your results, you'll need a CROSS JOIN:
SELECT u.NoEmploye, u.FirstName, u.LastName, Total=sum(h.NbHeures), ac.Name
FROM ActivityCategory ac
CROSS JOIN Users u
LEFT JOIN ActivityCodes code
ON code.Categorie=ac.Id
LEFT JOIN Heures h
ON h.Code=code.ActivityId
LEFT JOIN HeuresProjets hp
ON hp.HPId=h.HpGuid
LEFT JOIN Semaines s
ON s.Id=hp.WeekId AND u.EntityGuid=s.UserGuid
GROUP BY u.NoEmploye, u.FirstName, u.LastName, ac.Name
ORDER BY u.NoEmploye
To solve the issue when you want to add a WHERE clause:
SELECT u.NoEmploye, u.FirstName, u.LastName,
Total=COALESCE(SUM(h.NbHeures),0), ac.Name
FROM ActivityCategory ac
CROSS JOIN Users u
LEFT JOIN
ActivityCodes code
JOIN Heures h
ON h.Code=code.ActivityId
JOIN HeuresProjets hp
ON hp.HPId=h.HpGuid
JOIN Semaines s
ON s.Id=hp.WeekId
AND (some condition on the dates) -- add here
ON ac.Id = code.Categorie
AND u.EntityGuid = s.UserGuid
GROUP BY u.NoEmploye, u.FirstName, u.LastName, ac.Name
ORDER BY u.NoEmploye