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 //這里dc就相當於d from dci in dc.DefaultIfEmpty() //這里dci就相當於dc，所以dci就相當於d,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 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。