Doing 2 Left Joins on the same table with one call

Question

I have the following tables:

Users:

• userId, userFirstName, userLastName.

holdBilling:

• bEntityID, CarrierOID, PayerOID, holdTYPE, createUserID.

carrier:

• carrierOID, carrierName.

payer:

• payerOID, payerName.

I want the code to save in a new entity

holdBilling => new
{
      FirstName, LastName, CarrierName, PayerName
}

One of these Entities has either a payer or a carrier value (cannot have both). Basically i want to make 2 left joins on the same table with one call. This would be the SQL query that would work for me.

SELECT TOP 1000 [ID]
      ,[bEntityID]
      ,c.carrierID
      ,c.carrierName
      ,p.payerID
      ,p.payerName
      ,[holdType] ( this is "C" for carrier and "P" for payer ) 
  FROM .[dbo].[holdBilling] hb
  left join dbo.payer p on hb.payerID = p.payerID
  left join dbo.carrier c on hb.carrierID = c.carrierID
  where [bEntityID] = 378

The temporary solution I've found is getting a list of all Carriers

 var listC = (from hold in holdBilling
              join u in Users on hold.createUserID equals u.userID
              join c in carrier.DefaultIfEmpty() on hold.carrierID equals c.carrierID
                             select new
                             {
                                 Elem = hold,
                                 FName = u.userFirstName,
                                 LName = u.userLastName,
                                 Carrier = c.carrierName,
                                 Payer = ""
                             }).ToList();

and one for all payers

 select new
        {
            Elem = hold,
            FName = u.userFirstName,
            LName = u.userLastName,
            Carrier = "",
            Payer = p.payerName
        }).ToList();

and merging the two,I'm confident there has to be a solution for doing both in one query.

Answer 1

Something like this maybe:

var listC = (
                from hb in holdBilling
                from p in payer.Where(a=>a.payerID==hb.payerID).DefaultIfEmpty()
                from c in carrier.Where(a=>a.carrierID=hb.carrierID).DefaultIfEmpty()
                where hb.bEntityID==378
                select new
                {
                    hb.bEntityID,
                    c.carrierID,
                    c.carrierName,
                    p.payerID,
                    p.payerName,
                    holdType=(payer==null?"P":"C")
                }
                ).Take(1000)
                .ToList();

Answer 2

You need to use DefaultIfEmpty to do a left join

 var listC = (from hold in holdBilling
              from u in Users.Where(x => hold.createUserID == x.userID).DefaultIfEmpty()
              from c in carrier.Where(x => hold.carrierID == x.carrierID).DefaultIfEmpty()
              select new
              {
                Elem = hold,
                FName = u.userFirstName,
                LName = u.userLastName,
                Carrier = c.carrierName,
                Payer = ""
              }).ToList();