I'm trying to get a list of all users that have published blog posts within a given category. I'm using the Users Viewer web part. I had no trouble pulling only users that had blog posts with the following query:

UserID IN(
    SELECT NodeOwner FROM View_CONTENT_BlogPost_Joined WHERE 
    ClassName = 'CMS.BlogPost' AND 
    Published = 1 AND
    NodeAliasPath LIKE '/Blog/%'
)

But I now want to only display users that have blog posts in a given category, I thought I could modify my query to the following:

UserID IN (
    SELECT B.NodeOwner FROM View_CONTENT_BlogPost_Joined B
    JOIN CMS_DocumentCategory C ON B.BlogPostID = C.DocumentID
    WHERE 
    B.ClassName = 'CMS.BlogPost' AND 
    B.Published = 1 AND
    B.NodeAliasPath LIKE '/Blog/%' AND
    C.CategoryID = '{%categoryid%}'
)

However that is not returning any results. It's hard to pin down exactly what I'm missing because I don't get any SQL errors. Any thoughts? Am I looking at the totally wrong table? Thanks!