如何在EF中實現left join（左聯接）查詢

在EF中，當在dbset使用join關聯多表查詢時，連接查詢的表如果沒有建立相應的外鍵關系時，EF生成的SQL語句是inner join（內聯），對於inner join，有所了解的同學都知道，很多時候這並不是我們的本意，實例如下：

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生成了內連接（inner join）查詢，當兩個表的任一表的數據不匹配時，查詢結果就為空！實際上left join（左聯接）才是我們想要的，那么怎么樣才能生成left join查詢呢？其實只要我們如下改造，EF就能為我們生成left join（左聯接）查詢！

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
                       };                      

對比上下兩種寫法，可以看到在on表的后面我們加上了into xx，還有不要忘記，還需加上from xxx in xx.DefaultIfEmpty(),重要的就是最后的xx.DefaultIfEmpty(),它的作用是當連接的表為空時也會有一條空的數據，達到了left join的效果。