當使用EF中的dbset進行join操作的時候如果，如果表之間沒有關聯關系的話，生成的sql語句是 inner join（inner join 和 left join的區別 就不在這闡述了），eg：

var user = await (from u in this.context.MUser.Where(sa => sa.Id == pid)
                  join uw in this.context.MUserWechat on u.Id equals uw.UserID 
                  select new MVUserBaseInfo
                  {
                      ID = u.Id,
                      CreateTime = u.CreateTime,
                      HeadImage = u.HeadImage,
                      Phone = u.Phone,
                      UserName = u.UserName,
                      WechatBindStatus = uwti.WechatBindStatus,
                      WechatSystem = uwti.WechatSystem,
                      WechatUser = uwti.WechatUser,
                      NickName = uwti.NickName ?? string.Empty
                  }).FirstOrDefaultAsync();
 

這樣相當於就是將MUser和MUserWechat 進行了inner join， 如果我們希望進行左連接的話可以借助DefaultIfEmpty() 來達到當聯接的表為空的時候，生成一條空的數據，從而達到左聯接的效果，eg：

var user = await (from u in this.context.MUser.Where(sa => sa.Id == pid)
                  join uw in this.context.MUserWechat on u.Id equals uw.UserID into uwt
                  from uwti in uwt.DefaultIfEmpty() // 變成left join
                  select new MVUserBaseInfo
                  {
                      ID = u.Id,
                      CreateTime = u.CreateTime,
                      HeadImage = u.HeadImage,
                      Phone = u.Phone,
                      UserName = u.UserName,
                      WechatBindStatus = uwti.WechatBindStatus,
                      WechatSystem = uwti.WechatSystem,
                      WechatUser = uwti.WechatUser,
                      NickName = uwti.NickName ?? string.Empty
                  }).FirstOrDefaultAsync();
 

在EF中實現左聯接