How to use distinct with group by in Linq to SQL

Question

I'm trying to convert the following sql to Linq 2 SQL:

select groupId, count(distinct(userId)) from processroundissueinstance 
group by groupId

Here is my code:

var q = from i in ProcessRoundIssueInstance
    group i by i.GroupID into g
    select new
    {
        Key = g.Key,
        Count = g.Select(x => x.UserID).Distinct().Count()
    };

When I run the code, I keep getting Invalid GroupID. Any ideas? Seems the distinct is screwing things up..

Here is the generated sql:

SELECT [t1].[GroupID] AS [Key], (
SELECT COUNT(*)
FROM (
    SELECT DISTINCT [t2].[UserID]
    FROM [ProcessRoundIssueInstance] AS [t2]
    WHERE (([t1].[GroupID] IS NULL) AND ([t2].[GroupID] IS NULL)) 
       OR (([t1].[GroupID] IS NOT NULL) 
            AND ([t2].[GroupID] IS NOT NULL) 
            AND ([t1].[GroupID] = [t2].[GroupID]))
    ) AS [t3]
) AS [Count]
FROM (
    SELECT [t0].[GroupID]
    FROM [ProcessRoundIssueInstance] AS [t0]
    GROUP BY [t0].[GroupID]
    ) AS [t1]

Solution

I think Basiclife is close, but checking if the id is empty may not be the issue or enough, you should check to make sure it is not null before doing the group since you said it is a nullable field. Otherwise it looks right, and if you are having issues you may have bad data, or it is a bug or not fully implemented feature of Linq to SQL, and you may want to try Linq to Entity.

var q = from i in ProcessRoundIssueInstance
        where i.GroupID != null
        && i.GroupID != string.Empty
        group i by i.GroupID into g        
        select new
        {
            Key = g.Key,
            Count = g.Select(x => x.UserID).Distinct().Count()
        };

Comments

[Marco]

Yes, please look at my original post to see the invalid sql generated.

[Basic]

Out of interest, is there any Id column nullable intentionally? Also, can you tell us the data type for ProcessRoundIssueInstance according to the debugger (not declaration). Thanks

[Marco]

Yes, the GroupID is nullable but I also tried on another non-nullable column. System.Data.Linq.Table<ProcessRoundIssueInstance>