LINQ左外连接

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左连接或左外连接：包含左边的表的所有行，如果右边表中某行没有匹配，该行内容为空NULL。

SQL语句select * from dbo.Project left join dbo.Voice on (dbo.Project.voiceID=dbo.Voice.ID)

看一下网上的LINQ语句例子：左连接

var userInfo=from s in db.Project

join c in db.Voice on s.voiceID equals c.ID into ProjectV

from pv in ProjectV.DefaultEmpty() //???

select new {s.Name,s.bak,pv.NAME,s.ID};

我实在是不知道pv代表的什么意思？

再来对比一下例子。没有左外连接

list = (from u in dbcontext.t_announce                                join a in dbcontext.t_user on u.userID equals a.userID                                join b in dbcontext.t_type on u.typeID equals b.type_ID                                join u2 in dbcontext.t_user on u.memberID equals u2.userID                                join u3 in dbcontext.t_user on u.operateID equals u3.userID                                                                 where (u.departmentID == announce.departmentID && u.typeID == announce.typeID)                                orderby u.date descending                                select new GainViewModel()                                {                                    content = b.type_Name,                                    date = u.date,                                    usernamea = a.userName,                                    states = u.announceType,                                    dayGetID = u.announceID,                                    score = u.score,                                    shenqingren=u2.userName,                                    userNamec = u3.userName //审批人                                }).ToList();

有左外连接

   list = (from u in dbcontext.t_announce                                join a in dbcontext.t_user on u.userID equals a.userID                                join b in dbcontext.t_type on u.typeID equals b.type_ID                                join u2 in dbcontext.t_user on u.memberID equals u2.userID                                join u3 in dbcontext.t_user on u.operateID equals u3.userID into JoinedEmpDept                                  from u3 in JoinedEmpDept.DefaultIfEmpty()                                where (u.departmentID == announce.departmentID && u.typeID == announce.typeID)                                orderby u.date descending                                select new GainViewModel()                                {                                    content = b.type_Name,                                    date = u.date,                                    usernamea = a.userName,                                    states = u.announceType,                                    dayGetID = u.announceID,                                    score = u.score,                                    shenqingren=u2.userName,                                    userNamec = u3.userName //审批人                                }).ToList();

关键在这

 join u3 in dbcontext.t_user on u.operateID equals u3.userID into JoinedEmpDept                                  from u3 in JoinedEmpDept.DefaultIfEmpty()

效果

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