ROQL: How to use CategoryHierarchy

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I have this:

select Category.ID,Category.LookupName, COUNT(ID) as Total from Incident WHERE Source=1017 group by Category.LookupName order by Category.ID LIMIT 250

Counts incidents which have been a webchat organised by category. However in some of these situations, they do have a parent category. How would I display that? How would I expose CategoryHierarchy?