Linq 三表 left join 的实现

Posted 2021-02-12 sjwudhwhhw

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目的实现:select id,name,jname,cname from userinfo u left join job j on u.job=j.jid left join city c on u.city=c.cid  多表left join

                        var list = (from u in dc.userinfos
                        join j in dc.jobs on u.job equals j.jid into j_join
                        from x in j_join.DefaultIfEmpty()
                        join c in dc.cities on u.city equals c.cid into c_join
                        from v in c_join.DefaultIfEmpty()
                        select new
                        {
                            id=u.id,
                            name=u.name,
                            jname=x.jname,
                            cname=v.cname,
                           /*u1=u,x1=x,v1=v*///不要用对象的方式 因为对象可能为null那么对象.属性就会抛异常
                        }).ToList();

            for (var i = 0; i < list.Count(); i++)
            {
                Console.WriteLine(list[i].name+‘ ‘+list[i].jname+‘ ‘+list[i].cname); //字段为null不跑异常
                //Console.WriteLine(list[i].u1.name+‘ ‘+list[i].x1.jname+‘ ‘+list[i].v1.cname+" "); //对象x1 v1 有可能为null 抛异常
            }
                Console.ReadLine();

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