Question

Why Microsoft uses stupid examples with GroupBy clause only for one table?

http://msdn.microsoft.com/en-us/library/bb896341.aspx

And why error is causing when i tried to Select some field that is not in GroupBy?
Error says that fields that is not in GroupBy clause doesn’t exist in current context.

using (Entities db = new Entities())
{
    var query = "SELECT RLS.RoleId, UIR.UserId, UIR.RoleId, USR.UserName FROM Entities.aspnet_Roles AS RLS " +
                 "INNER JOIN Entities.vw_aspnet_UsersInRoles AS UIR " +
                 "ON RLS.RoleId = UIR.RoleId " + 
                 "INNER JOIN Entities.aspnet_Users AS USR " + 
                 "ON UIR.UserId = USR.UserId " + 
                 //"GROUP BY RLS.RoleId"  // is uncomment - it says UserId doesn't exist in context
                 ;
    var x = new ObjectQuery<DbDataRecord>(query, db);
    var y = x.ToTraceString().Replace("\n", " ").Replace("\t", " ").Replace("\r", " ");
}

The same for LINQ:

var x = from RLS in db.aspnet_Roles
                        join URS in db.vw_aspnet_UsersInRoles
                        on RLS.RoleId equals URS.RoleId
                        join USR in db.aspnet_Users
                        on URS.UserId equals USR.UserId
                        group RLS by RLS.RoleId into GRP
                        select new
                        {
                            GRP.Key
                        };

The question is: how to select all fields and GroupBy selection by only one field?

Thanks in advance.

Answer 1

All fields that are not in the GROUP BY clause have to be aggregate expressions such as AVG(...) or SUM(...) or MAX(...) etc.

So this is wrong:

SELECT RLS.RoleId, UIR.UserId, UIR.RoleId, USR.UserName
(...)
GROUP BY RLS.RoleId

because the last three expressions in the SELECT should either be aggregated somehow or added to the GROUP BY clause.

This is logical because by specifying GROUP BY RLS.RoleId you say that RLS.RoleId should be unique in the end result, so only one row per value. But for each RLS.RoleID there could be multiple values for UIR.UserId, UIR.RoleId or USR.UserName. So you need to tell the DBMS what to do to make them into one row: average, maximum, minimum… OR you add fields to the GROUP BY clause, so that not RLS.RoleID but the combination of column values should be unique. See also this example.