在EF中，当在dbset使用join关联多表查询时，连接查询的表如果没有建立相应的外键关系时，EF生成的SQL语句是inner join（内联），对于inner join，有所了解的同学都知道，很多时候这并不是我们的本意，实例如下：

var list = from o in context.CTMS_OD_ORDERS
                       join d in context.CTMS_SUP_DOCTOR
                       on o.OWNERDOCID equals d.USERID
                       join e in context.CTMS_OD_ORDERSEVALUATION
                       on o.ORDERID equals e.ORDERID
                       select o;`

EF生成了内连接（inner join）查询，当两个表的任一表的数据不匹配时，查询结果就为空！实际上left join（左联接）才是我们想要的，那么怎么样才能生成left join查询呢？其实只要我们如下改造，EF就能为我们生成left join（左联接）查询！

data = from o in context.CTMS_OD_ORDERS
                       join d in context.CTMS_SUP_DOCTOR 
                       on o.OWNERDOCID equals d.USERID into dc
                       from dci in dc.DefaultIfEmpty()
                       join e in context.CTMS_OD_ORDERSEVALUATION
                       on o.ORDERID equals e.ORDERID into ec
                       from eci in ec.DefaultIfEmpty()
                       where o.USERID == userID && (string.IsNullOrEmpty(type) || o.PRODUCTNAME.Contains(type))
                       select new ODOrders
                       {
                           BalanceStatus = o.BALANCESTATUS,
                           ChannelOrderID = o.CHANNELORDERID,
                           ChannelType = o.CHANNELTYPE,
                           CreateDateTime = o.CREATEDATETIME,
                           CreateUserID = o.CREATEUSERID,
                           CreateUserName = o.CREATEUSERNAME,
                           DocName = dci.DOCNAME,
                           EvalutionStatus = string.IsNullOrEmpty(eci.ORDERID) ? "0" : "1",
                           PayTime = o.PAYTIME,
                           ProductCode = o.PRODUCTCODE,
                           ProductName = o.PRODUCTNAME,
                           ProductInstanceId = o.PRODUCTINSTANCEID,
                           ProductID = o.PRODUCTID,
                           OrderID = o.ORDERID,
                           OrderCode = o.ORDERCODE,
                           OrderStatus = o.ORDERSTATUS,
                           OrderType=o.ORDERTYPE,
                           TotalFee = o.TOTALFEE,
                           UserID=o.USERID,
                           UserName=o.USERNAME
                       };                      

对比上下两种写法，可以看到在on表的后面我们加上了into xx，还有不要忘记，还需加上from xxx in xx.DefaultIfEmpty(),重要的就是最后的xx.DefaultIfEmpty(),它的作用是当连接的表为空时也会有一条空的数据，达到了left join的效果。