使用Entity Framework Core从数据库记录的最大和最小

Question

I have a simple method that return rage of data from db according to some parameters, and it is simple:

public async Task<IEnumerable<P2PStats>> GetFilteredNetStats(ushort id, ushort remoteId, DateTime start, DateTime end)
{
    using (var ctx = new DataContext())
    {
        IQueryable<P2PStats> query = ctx.P2PStats
             .Where(stat => stat.Id == id && stat.Date >= start && stat.Date <= end)
             .Where(stat => stat.P2PStatsDetailed.Any(detail => detail.RemoteId == remoteId))
             .Select(stat => new P2PStats
             {
                 Id = stat.Id,
                 AxmCardId = stat.Id,
                 Date = stat.Date,
                 P2PStatsDetailed = stat.P2PStatsDetailed.Where(detail => detail.RemoteId == remoteId).ToList()
             });

        return await query.ToListAsync();
    }
}

It returns collection of P2PStats(well actually a task but in final result a collection). Can this be modified so that I can get only 2 values from database first with lowest date second with highest?

I tried Max and Min but only after the query has been materialized and I end up with max and min value or property not whole record.

Answer 1

This problem can be solved by the removal of filtering by Id and the addition of the method .Max() and .Min() to your code.

Below is an example of how this would look:

 public async Task<IEnumerable<P2PStats>> GetNetStatsLowestAndHighestDate(ushort id, ushort remoteId, DateTime start, DateTime end)
{
    using (var ctx = new DataContext())
    {
        IQueryable<P2PStats> query = ctx.P2PStats
             .Where(stat => stat.Date >= start && stat.Date <= end)
             .Where(stat => stat.P2PStatsDetailed.Any(detail => detail.RemoteId == remoteId))
             .DefaultIfEmpty(0)
             .Max(s => s.Date)

             .Select(stat => new P2PStats
                                        {
                                            Id = stat.Id,
                                            AxmCardId = stat.Id,
                                            Date = stat.Date,
                                            P2PStatsDetailed = stat.P2PStatsDetailed.Where(detail => detail.RemoteId == remoteId).ToList()
                                        });

        IQueryable<P2PStats> query2 = ctx.P2PStats
             .Where(stat => stat.Date >= start && stat.Date <= end)
             .Where(stat => stat.P2PStatsDetailed.Any(detail => 
        detail.RemoteId == 
        remoteId))
             .DefaultIfEmpty(0)
             .Min(s => s.Date)

             .Select(stat => new P2PStats
                                        {
                                            Id = stat.Id,
                                            AxmCardId = stat.Id,
                                            Date = stat.Date,
                                            P2PStatsDetailed = 
        stat.P2PStatsDetailed.Where(detail => detail.RemoteId == remoteId).ToList()
                                        });

        var results = query1.Concat(query2);

        return await results.ToListAsync();

    }
}

Answer 2

i assume you have records for the same ID. just created a linqpad example (i was not sure if you want list of min value, max value rows or just min and max) the approach would be to group on same field (propably on ID) and pick min, max

void Main()
{
    test t = new test();
    var l = new List<test>() {
        new test() {ID = 0, a1="aaa", a2 = 10},
        new test() {ID = 1, a1="aaa", a2 = 40},
        new test() {ID = 2, a1="aaa", a2 = 70},
        new test() {ID = 3, a1="aaa", a2 = 50},
        };

    l.Dump("original");
    l.GroupBy(g => g.a1).Select(s => new { max = s.Max(mm => mm.a2), min = s.Min(mi => mi.a2) }).Dump("return 2 values");

    List<test> lRes = new List<test>();
    lRes.Add(l.OrderBy(o => o.a2).First());
    lRes.Add(l.OrderBy(o => o.a2).Last());
    lRes.Dump("return table of min record and max record");


}

public class test
{
    public int ID { get; set; }
    public string a1 { get; set; }
    public int a2 { get; set; }
    public test() { }
}