Question

I’m looking for a way to get the Entity Framework to return one record for every minute in a database field e.g. for the table below:

Message | Timestamp
--------+--------------------
a       | 2011-01-02 12:31:10
b       | 2011-01-02 12:31:15
c       | 2011-01-02 12:31:59
d       | 2011-01-02 12:32:01
e       | 2011-01-02 12:32:30
f       | 2011-01-02 12:33:10

I want a query that returns

c,e,f (c and e are selected because they are the latest entries)

Is this possibly with a single query? The database is quite large so I don’t want to retrieve all records and select them in a loop.

Answer 1

I use Linq for an in-memory collection instead of EF, but you should be able to adapt this easily.

    private static void LinqExample()
    {
        var a = new object[]
                    {
                        new object[] {"a", DateTime.Parse(" 2011-01-02 12:31:10")},
                        new object[] {"b", DateTime.Parse(" 2011-01-02 12:31:15")},
                        new object[] {"c", DateTime.Parse(" 2011-01-02 12:31:59")},
                        new object[] {"d", DateTime.Parse(" 2011-01-02 12:32:01")},
                        new object[] {"e", DateTime.Parse(" 2011-01-02 12:32:30")},
                        new object[] {"f", DateTime.Parse(" 2011-01-02 12:33:10")},
                    };

        var result = from pair in a
                     let msg = (string) ((object[]) pair)[0]
                     let date = (DateTime) ((object[]) pair)[1]
                     group new {date, msg} by new {date.Year, date.Month, date.Day, date.Hour, date.Minute}
                     into dateGroup
                     select dateGroup.OrderBy(dg => dg.date).Last();

        foreach (var res in result)
        {
            Console.WriteLine(res.msg);
        }
    }