Question

I am trying and so far not succeeding in making a query with a group by-statement.

I have a table with different types of activities (tblExcursion) and a table with the planned (tblReservationDetail). In tblExcursion I have a column ‘GroupName’. There are multiple activities under one GroupName (e.g. kayak, sailing, waterpolo is groupname Watersports).

Now I want to retrieve the number of participants per GroupName (not per activity/excursion!). But I am not getting there with .GroupBy() or group…by

This is the query where I use the activityname and not the groupname:

var vAllExcursions = (from oExcursion in clsApplication._oDBConnection.tblExcursions
                      select oExcursion).ToList();

foreach (tblExcursion EXitem in vAllExcursions)
{
    var vAllExcursionsPerType = (from oReservationDetail in clsApplication._oDBConnection.tblReservationDetails
                                         where oReservationDetail.StartTime.Date.Year == this.cboYear.getSelectedID()
                                         && oReservationDetail.StartTime.Date.Month == this.cboMonth.getSelectedID()
                                         && oReservationDetail.ExcursionID == EXitem.ID
                                         select oReservationDetail).ToList();
}

Here I fill my datagrid

Answer 1

You want to group by the GroupName to get the groups:

var vAllExcursions = clsApplication._oDBConnection.tblExcursions
    .GroupBy(ex => ex.GroupName)
    .ToList();

Then in your inner loop, find any related reservations by checking if the ExcursionID matches any id in the list associated with the group:

foreach (var EXitemGroup in vAllExcursions)
{
    var excursionIds = EXitemGroup.Select (exg => exg.ID).ToArray();
    ...
        && excursionIds.Contains(oReservationDetail.ExcursionID)
    ...
}