Question

That is the most efficient way to perform a left outer join in LINQ if I must do the following…

• Filter Table 2 by a beginning and ending date.
• All rows in Table 1 must remain, even if the filtering of Table 2 returns no rows.
• The result must be grouped so that the columns from Table 2 get summed.

For example (example code variable names changed for propietary reasons), supposed I have a database with two tables. Table 1 has a list of doors with a building code, door ID and current status (open or closed) – the building code and door ID are the primary key. Table 2 has a list of events for all doors (an event is an opening or closing) plus a timestamp. So the columns are building code, door ID, timestamp, opening, closing. Opening and closing are integers with a 1 in the column for the appropriate event. There is a foreign key relationship between the two tables on the building code and door ID.

For my query I need to return a list of all the unique doors with the current door status and a sum of all the opening and closing events for a selected time period. An entry must be returned for each door, even if no events occured during the selected time period.

Below is the best LINQ code I could come up with. It works, but it seems really inefficient and hard to understand. How would you make it more efficient and easier to understand?

var query = 
    from doors in Context.Doors
    join fevents in
        (
        from events in db.Events
        where events.TimeStamp >= date1 && events.TimeStamp <= date2
        select new { events.BuildingCode, events.DoorID, events.TimeStamp, events.Opening, events.Closing }
        )
    on new { doors.BuildingCode, doors.DoorID } equals { fevents.BuildingCode, fevents.DoorID }
    into g1
    from c in g1.DefaultIfEmpty()
    group c by new
    {
        doors.BuildingCode,
        doors.DoorID,
        doors.DoorStatus
    } into g2
    select new
    {
        BuildingCode = g2.Key.BuildingCode,
        DoorID = g2.Key.DoorID,
        Status = g2.Key.DoorStatus
        NumOpenings = g2.Sum(i => (i == null ? 0 : i.Opening)),
        NumClosings = g2.Sum(i => (i == null ? 0 : i.Closing))
    };

Answer 1

I think this is slightly easier to read

var query = 
    from doors in Context.Doors
    from c in db.Events
                .Where(events => doors.BuildingCode == events.BuildingCode)
                .Where(events => doors.DoorID == events.DoorID)
                .Where(events => events.TimeStamp >= date1 && events.TimeStamp <= date2)
                .Select(events => new { events.BuildingCode, events.DoorID, events.TimeStamp, events.Opening, events.Closing })
                .DefaultIfEmpty()
    group c by new
    {
        doors.BuildingCode,
        doors.DoorID,
        doors.DoorStatus
    } into g2
    select new
    {
        BuildingCode = g2.Key.BuildingCode,
        DoorID = g2.Key.DoorID,
        Status = g2.Key.DoorStatus
        NumOpenings = g2.Sum(i => (i == null ? 0 : i.Opening)),
        NumClosings = g2.Sum(i => (i == null ? 0 : i.Closing))
    };