Linq 查询：按子查询使用 count() 组进行内部连接

Question

We have some Machines having Devices attached. Not all the machines have devices atached and the devices cand be moved between machines. The devices generate Errors and we need to count those errors occured in the past day.

We have four tables: Machines (with Id and Code), Devices (with Id and Code), a pairing table DevicesMachines (Id, IdMachine, IdDevice, From datetime, To datetime) and Errors(Id, IdDevice, Moment datetime, Description).

The working SQL query is this:

Select m.Id, m.Code, 
   Coalesce(d.Code, 'NA') As DeviceCode,
   Coalesce(Err.ErrorCnt,0) As ErrorCnt
From Machines As m
Left Outer Join (Select IdMachine, IdDevice From DevicesMachines as dm 
Where GetDate() Between dm.From And dm.To) As dm on m.Id=dm.IdMachine
Left Outer Join Devices As d on dm.IdDevice=d.Id
Left outer join 
  ( Select IdMachine, Count(Id) As ErrorCnt From Errors as er 
    Where er.Moment >= DateAdd(day,-1,GetUtcDate()) 
    Group By IdMachine) As Err 
On m.Id=Err.IdMachine

I have tried many syntaxes, one of which is below:

using ( DataContextM dcMachines = new dataContextM())
{
  IEnumerable<MachineRow> lstM = 
    from m in dcMachines.Machines
    from dm in dcMachines.DevicesMachines.Where(dm => (dm.IdMachine == m.Id) && (dm.From <= DateTime.Now) && (dm.To >= DateTime.Now)).DefaultIfEmpty()
    from d in dcMachines.Devices.Where(d => d.Id == dm.IdDevice).DefaultIfEmpty()
    from er in dcMachines.Errors
        .Where(er => (er.Moment >= DateTime.Now) && (er.Moment <= DateTime.Now.AddDays(-1))) 
        .GroupBy(er => er.IdMachine)
        .Select(er => new { IdMachine = er.Key, ErrorCnt = er.Count() })
        .Where(er=> er.IdMachine==m.Id).DefaultIfEmpty()
 select new MachineRow
 {
    Id = amId,
    Code = m.Code,
    DeviceCode = (d == null) ? "NA" : d.DeviceCode,
    IdDevice = (d == null) ? 0: d.Id,
    ErrorCnt = (er == null) ? 0 : er.ErrorCnt
  };
}

I failed to find the right Linq syntax and I need your help.

Thank you, Daniel

Answer 1

Based on the SQL you provided, I created what I think is the equivalent EF LINQ expression:

using (var dcMachines = new DataContextM())
{
    var now = DateTime.Now;
    var utcYesterday = DateTime.UtcNow.AddDays(-1);

    var devicesMachinesQuery =
        from dm in dcMachines.DevicesMachines
        where dm.From <= now && dm.To >= now
        join d in dcMachines.Devices on dm.IdDevice equals d.Id into dItems
        from d in dItems.DefaultIfEmpty()
        select new
        {
            dm.IdMachine,
            dm.IdDevice,
            DeviceCode = d != null ? d.Code : "NA"
        };

    var errorsQuery =
        from err in dcMachines.Errors
        where err.Moment >= utcYesterday
        select err;

    IEnumerable<MachineRow> lstM =
        from m in dcMachines.Machines
        join dm in devicesMachinesQuery on m.Id equals dm.IdMachine into dmItems
        from dm in dmItems.DefaultIfEmpty()
        select new MachineRow
        {
            Id = m.Id,
            Code = m.Code,
            DeviceCode = dm != null ? dm.DeviceCode : "NA",
            IdDevice = dm != null ? dm.IdDevice : 0,
            ErrorCnt = (
                from err in errorsQuery
                where err.IdMachine == m.Id
                select err.Id
                )
                .Count()
        };
}

I made some tests in memory and it seems to yield the same behavior as your provided SQL query.