Linq2Sql query - Pivot data with Grouping

I have the following dataset for a TimeTable that needs to be displayed in a gridview. Currently a snippet of the dataset looks like this:

SessionNum    TimeStart    TimeStop    Details
----------    ---------    --------    -------
1             08:00        09:00       Math101
1             09:00        10:00       Comp102
1             11:00        12:00       Engn101
2             08:00        09:00       Comp102
2             09:00        10:00       Math101
2             10:00        11:00       Acco103

There are a total of 5 sessions, and I would like for the dataset to look like:

TimeStart    TimeStop    Session1    Session2     ...
---------    --------    --------    --------     ---
08:00        09:00       Math101     Comp102
09:00        10:00       Comp102     Math101
10:00        11:00       -           Acco103
11:00        12:00       Engn101     -

As you will see, there are no aggregate functions required...just grouping, but for the life of me I cannot seem to wrap my head around this one. I have the following LINQ query which generates the first dataset:

List<TimeTable> list = db.Tim开发者_高级运维eTables.OrderBy(o => o.TimeStart).OrderBy(o => o.SessionNum).ToList();

This works fine, and generates the dataset sorted by SessionNum and then TimeStart. My attempt to solve this invlovled the following query:

var result = list.GroupBy(t => t.TimeStart).Select(s => new {
    TimeStart = s.Key,
    Session1 = s.Where(x => x.SessionNum == 1),
    Session2 = s.Where(x => x.SessionNum == 2)
});

This ran, but unfortunately did not work. I know a GroupBy (or a couple) is/are required, but I'm a bit lost from this point forward. I would really appreciate any help towards solving this. Thank you in advance!

---

You can't directly do a pivot query in LINQ. What you can do instead is create a structure like this:

var record = new
{
    TimeStart = "10:00",
    TimeStop = "11:00",
    Sessions = new [] { "-", "Acco103", },
};

When you have a list of these records you must ensure that the Sessions property is array that is the same length as the distinct number of sessions in your entire set of data. Then you can access the session information by indexing into the array.

This should make more sense after looking at the queries.

First, query the database for the required data:

var query =
    from s in db.TimeTables
    orderby s.TimeStop
    orderby s.TimeStart
    group s by new { s.TimeStart, s.TimeStop } into gss
    select new
    {
        gss.Key.TimeStart,
        gss.Key.TimeStop,
        Sessions = gss.ToArray(),
    };

Now determine the distinct set of sessions:

var sessionNums =
    db.TimeTables
        .Select(s => s.SessionNum)
        .Distinct()
        .OrderBy(n => n)
        .ToArray();

Now process this data in memory (note the .ToArray() call on query):

var process =
    from q in query.ToArray()
    let lookup = q.Sessions
        .ToLookup(s => s.SessionNum, s => s.Details)
    select new
    {
        q.TimeStart,
        q.TimeStop,
        Sessions = sessionNums
            .Select(n => String.Join(
                ", ",
                lookup[n].DefaultIfEmpty("-")))
            .ToArray(),
    };

This is where the hard work is. The lookup creates an easy way to get session detail out for any SessionNum. Calling lookup[n].DefaultIfEmpty("-") ensures that there is at least a single value for each session. The String.Join ensures that if the source data had two sessions for the same session number at the same time that we end up with one value.

This result is safe no matter how many sessions there are as it will just extend the arrays.

The output of the process query looks like this:

Then you can do this query:

var result =
    from p in process
    select new
    {
        p.TimeStart,
        p.TimeStop,
        Session1 = p.Sessions[0],
        Session2 = p.Sessions[1],
    };

This will effectively "pivot" your results, but you need to explicitly put in each "SessionX" property.

The output of the result query looks like this: