如何使用lambda表达式过滤多列的实体框架结果

Question

I have the following table:

And the following data:

How can i filter the result, so that i only get the latest row from each omraade_id (sorted descending by timestamp )?

Which in this case would be the rows with id: 1010 and 1005

--

From @lazyberezovsky's answer, i have created the following expression:

dbConnection = new ElecEntities();

            var query = from data in dbConnection.Valgdata
            orderby data.timestamp descending
            group data by data.omraade_id into g
            select g.FirstOrDefault();

            return query.ToList();

It returns two rows with the ID 3 and 4, which are the first two rows in the database, and also the ones with the lowest timestamp. Any idea why?

Answer 1

var query = dbConnection.Valgdata
                        .GroupBy(x => x.omraade_id)
                        .Select(g => g
                             .OrderByDescending(x => x.timestamp)
                             .FirstOrDefault());

Answer 2

I have no experience with EF, so I'm unsure if only SQL-esque linq works here. A plain C#-ish:

var query = dbConnection.Valgdata.GroupBy(u => u.omraade_id)
      .Select(x => x.FirstOrDefault(y => x.Max(p => p.timestamp) == y.timestamp));

Answer 3

var query = from v in dbConnection.Valgdata
            orderby v.timestamp descending
            group v by v.omraade_id into g
            select g.First();

This will return only record with max timestamp for each omraade_id.

UPDATE query above works fine to me (at least for MS SQL Linq provider). Also you don't need to do FirstOrDefault - if omraade_id is grouped, then it definitely has at least one row.

var query = from v in dbConnection.Valgdata
            group v by v.omraade_id into g
            select g.OrderByDesc(x => x.timestamp).First();

Answer 4

You have put filter on every item. It should be applied on complete query result, not on every item.

Following is updated query.

 var query = (from data in dbConnection.Valgdata
        orderby data.timestamp descending
        group data by data.omraade_id into g
        select g).FirstOrDefault();

Answer 5

This is my solution so far:

var data = dbConnection.Valgdata.Where(x => x.godkendt == false).ToList();
var dataGrouped = data.GroupBy(x => x.omraade_id).ToList();

List<Valgdata> list = new List<Valgdata>();

 foreach (var grpdata in dataGrouped)
            {
                var dataGroup = grpdata.OrderByDescending(x => x.timestamp).ToList();
                list.Add(dataGroup.FirstOrDefault());
            }
return list;

I dont know if it is the most effective, but it works.