var query=from a in A
          join b in B
          on A.a_id equals b.a_id
          where a.a_isdel==0 && b.b_isdel==0
          group b by b.a_id into g
          select new 
          {
              a_id=g.Key,
              a_name=A.SingleOrDefault(c=>c.a_id==g.Key).a_name,
              b_name=g.OrderBy(c=>c.b_id).FirstOrDefault().b_name
          };

//代码手写 谨提供思路参考

------解决方案--------------------
猫哥写的很好啊

------解决方案--------------------
var result = from u in db.TableA
            from n in db.TableB
            where u.a_id == n.a_id && u.a_isdel == 0 && n.b_isdel == 0
            group n by n.a_id into g
            select new
            {
                a_id = g.Key,
                a_name = db.TableA.SingleOrDefault(c => c.a_id == g.Key).a_name,
                b_name =g.FirstOrDefault(c=>c.b_id==(g.Min(t=>t.b_id))).b_name
            };

------解决方案--------------------
探讨


C# code

var query=from a in A
          join b in B
          on A.a_id equals b.a_id
          where a.a_isdel==0 && b.b_isdel==0
          group b by b.a_id into g
          select new 
 ……