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

Using SQL commands, parameters and subreports - CR

My report contains a number of subreports, each comprising a SQL command and using a parameter value passed from the main report.

1) Set up the main report parameter, in this instance called ?School

2) Add a subreport, Add Command with the Where clause as follows:

WHERE (School.SchoolKey = {?School})

3) For the Parameter list, Click Create:

Parameter Name = School
Value Type = Number

It will then prompt for a parameter value, enter 1 (anything will suffice at this stage)

NOTE: When the database field is a uniqueidentifier, use 'String' type and for the prompt use a value in the uniqueidenfier format: {29D2B899-393A-4592-9B00-D289884A1F94}

4) Back in the main report, right click the subreport and Change Subreport Links:

Available Fields = ?School
Subreport parameter field to use = ?School (make sure to select the user-generated one and not the automatically generated one named similar to ?PM-?School)

5) Voila! Main report parameters should now be passed to SQL Commands in subreports.