Question

I’ve been getting stuck into some linq queries for the first time today and I’m struggling with some of the more complicated ones. I’m building a query to extract data from a table to build a graph. The tables colums I’m interested in are Id, Time and Value.

The user will select a start time, an end time and the number of intervals (points) to graph. The value column will averaged for each interval.

I can do this with a linq request for each interval but I’m trying to write it in one query so I only need to go to the database once.

So far I have got:

var timeSpan = endTime.Subtract(startTime);
var intervalInSeconds = timeSpan.TotalSeconds / intervals;

var wattList = (from t in _table
                where t.Id == id
                    && t.Time >= startTime
                    && t.Time <= endTime
                group t by  intervalInSeconds // This is the bit I'm struggling with
                    into g
                    orderby g.Key 
                    select g.Average(a => a.Value))
                ).ToList();

Any help on grouping over time ranges will be most welcome.

Answer 1

I’ve done this myself for exactly the same situation you describe.

For speed, modified the database’s datapoints table to include an integer-based time column, SecondsSince2000, and then worked with that value in my LINQ to SQL query. SecondsSince2000 is a computed column defined as:

datediff(second, dateadd(month,1200,0), DataPointTimeColumn) PERSISTED

Where DataPointTimeColumn is the name of the column that stores the datapoint’s time. The magic function call dateadd(month,1200,0) returns 2000-01-01 at midnight, so the column stores the number of seconds since that time.

The LINQ to SQL query is then made much simpler, and faster:

int timeSlotInSeconds = 60;

var wattList = 
    (from t in _table
     where t.Id == id
           && t.Time >= startTime
           && t.Time <= endTime
     group t by t.SecondsSince2000 - (t.SecondsSince2000 % timeSlotInSeconds)
     into g
     orderby g.Key 
     select g.Average(a => a.Value))).ToList();

If you can’t modify your database, you can still do this:

var baseTime = new DateTime(2000, 1, 1);

var wattList = 
    (from t in _table
     where t.Id == id
           && t.Time >= startTime
           && t.Time <= endTime
     let secondsSince2000 = (int)(t.Time- baseTime).TotalSeconds
     group t by secondsSince2000 - (secondsSince2000 % timeSlotInSeconds)
     into g
     orderby g.Key 
     select g.Average(a => a.Value))).ToList();

The query will be quite a bit slower.