从SQL到LINQ（实现LEFT JOIN）

Question

我在SQL查询下面有一个查询，它比较复杂，有一些GroupBy，Conditions等。为了简化我的问题，我在这里跳过了以下内容：

SELECT 
    SUBSTRING(TSL.ctg_name, 6, 100) AS 'TREKK', *

    FROM Filteredctg_timbersettlementline AS TSL 

    INNER JOIN Filteredctg_timbersettlement AS TS 
        ON TSL.ctg_timbersettlementid = TS.ctg_timbersettlementid 
    LEFT JOIN FilteredNew_property AS P 
        ON TS.ctg_propertyid = P.new_propertyid

    WHERE (TS.ctg_timbersettlementid = @TimberSettlementID) AND
          (TSL.ctg_reportgroup = 'TREKK')

我尝试了以下Linq：

var Trekks = (from ts in XrmContext.ctg_timbersettlementSet

              join tsl in XrmContext.ctg_timbersettlementlineSet
                  on ts.Id equals tsl.ctg_timbersettlementid.Id
              join p in XrmContext.New_propertySet
                  on ts.ctg_propertyid.Id equals p.New_propertyId

              into temp
              from p in temp.DefaultIfEmpty()

              where ts.ctg_timbersettlementId == TimberSettlementGuid
              where tsl.ctg_reportgroup == "TREKK"
              select new
              {
                  tsl.ctg_name,
                  ts.ctg_BasisAllocatedForestryFund,
              }).ToList();

它扔我一个错误：方法“群组加入”不能跟随法“加入”或不被支持。 尝试使用受支持的方法编写查询，或者在调用不受支持的方法之前调用“ AsEnumerable”或“ ToList”方法。

我怎样才能达到LEFT JOIN？

Answer 1

要实现左连接，您需要使用中介DefaultIfEmpty（）：

var query = from c in db.Customers
            join o in db.Orders
               on c.CustomerID equals o.CustomerID into sr
            from x in sr.DefaultIfEmpty()
            select new {
               CustomerID= c.CustomerID, ContactName=c.ContactName,
               OrderID = x.OrderID == null ? -1 : x.OrderID};