在EF中实现左联接

Posted 2021-02-05 penz

当使用EF中的dbset进行join操作的时候如果，如果表之间没有关联关系的话，生成的sql语句是 inner join（inner join 和 left join的区别 就不在这阐述了），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();

这样相当于就是将MUser和MUserWechat 进行了inner join， 如果我们希望进行左连接的话可以借助DefaultIfEmpty() 来达到当联接的表为空的时候，生成一条空的数据，从而达到左联接的效果，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() // 变成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();