Count Distinct Over Partition

To get a count of distinct values over a windowing partition:

DENSE_RANK() OVER (PARTITION BY e.Dept_Desc order by e.Staff_No)
+ DENSE_RANK() OVER (PARTITION BY e.Dept_Desc order by e.Staff_No desc)
- 1

Using Count function on a uniqueidentifier field - SQL

SELECT COUNT(D.DOCUMENTID) AS DocCount, P.TITLE
FROM DOCUMENT AS D inner join
PROJECT AS P ON D.PROJECTID = P.PROJECTID
GROUP BY P.PROJECTID, P.TITLE

When running this query I encountered the following error:

Msg 409, Level 16, State 2, Line 1
The count aggregate operation cannot take a uniqueidentifier data type as an argument.

   
To avoid this error when trying to Count a uniqueidetifier field you have to CAST it into a char :
 
SELECT COUNT(CAST(D.DOCUMENTID AS char(36))) AS DocCount, P.TITLE
FROM DOCUMENT AS D inner join
PROJECT AS P ON D.PROJECTID = P.PROJECTID
GROUP BY P.PROJECTID, P.TITLE