How can I merge the results of a group by Linq-to-XML query?

IEnumerable<GroupItem> linqQuery = detailList
    .GroupBy(i => i.A)
    .Select(g => new GroupItem() 
    { 
        A = g.Key, 
        AllBsWithinA = g.Select(i => i.B).ToList() 
    });

var result = Items.GroupBy(x => new { x.Size, x.Type })
                  .Select(x => new
                    {
                        Key = x.Key,
                        Ids = x.Select(r => r.Id)
                    });

public class Stock {
    public string Name {get; set;}
    public List<int> Values {get; set;}
    public float Price {get; set;}
    public override bool Equals(object obj) {
        if (obj == this) return true;
        var other = obj as Stock;
        if (other == null) return false;
        return Name.Equals(other.Name)
            && Price == other.Price
            && Values.SequenceEqual(other.Values);
    }
    public override int GetHashCode() {
        return Name.GetHashCode()
             + Price.GetHashCode()
             + Values.Sum();
    }
}

DataRow[] dr1 = dtt.Select("Course = 'Math' AND Month > 10");
        var result_one = dr1.AsEnumerable()
            .GroupBy(r => new { Name = r.Field<string>("Name"), Class = r.Field<string>("Class") })
            .Select(g => new
            {
                Name = g.Key.Name,
                Class = g.Key.Class,
                Max = g.Max(r => r.Field<int>("Score")),
                Max_Month = g.FirstOrDefault(gg => gg.Field<int>("Score") == g.Max(r => r.Field<int>("Score"))).Field<int>("Month"),
            }
            ).Distinct().ToList();


DataRow[] dr2 = dtt.Select("Course = 'Chem'");
        var result_two = dr2.AsEnumerable()
            .GroupBy(r => new { Name = r.Field<string>("Name"), Class = r.Field<string>("Class") })
            .Select(g => new
            {
                Name = g.Key.Name,
                Class = g.Key.Class,                    
                Chem_Max = g.Max(r => r.Field<int>("Score")),
                Chem_Max_Month = g.FirstOrDefault(gg => gg.Field<int>("Score") == g.Max(r => r.Field<int>("Score"))).Field<int>("Month"),
            }
            ).Distinct().ToList();

var lstLeftJoin = (from a in result_one
                           join b in result_two
                           on new { a.Name, a.Class } equals new { b.Name, b.Class }
                           into gj
                           from subpet in gj.DefaultIfEmpty()
                           select new { a.Name, a.Class, Math_Max_Month = a.Max_Month, Math_Max = a.Max, Chem_Max_Month = (subpet == null ? 0 : subpet.Chem_Max_Month), Chem_Max = (subpet == null ? 0 : subpet.Chem_Max) }).ToList();

  var lstRightJoin = (from a in result_two
                            join b in result_one
                            on new { a.Name, a.Class } equals new { b.Name, b.Class }
                            into gj
                            from subpet in gj.DefaultIfEmpty()
                            select new { a.Name, a.Class, Math_Max_Month = (subpet == null ? 0 : subpet.Max_Month), Math_Max = (subpet == null ? 0 : subpet.Max), a.Chem_Max_Month, a.Chem_Max }).ToList();

var lstUnion = lstLeftJoin.Select(s => new { Name = s.Name, Class = s.Class, Math_Max_Month = s.Math_Max_Month, Math_Max = s.Math_Max, Chem_Max_Month = s.Chem_Max_Month, Chem_Max = s.Chem_Max }).Union(lstRightJoin.Select(s => new { Name = s.Name, Class = s.Class, Math_Max_Month = s.Math_Max_Month, Math_Max = s.Math_Max, Chem_Max_Month = s.Chem_Max_Month, Chem_Max = s.Chem_Max })).OrderBy(o => o.Name).ThenBy(c => c.Class).ToList();

Name  Class    Math_Max_Month    Math_Max  Chem_Max_Month   Chem_Max
Alex   C1      12                90        null             null
Alex   C2      11                91        12               92
Bob    C1      null              null      12               97
Bob    C2      12                94        null             null

var BooksList = from Book in BookTable.AsEnumerable()
                        group Book by Book.Field<string>("BookCategory") into g
                        let list = g.ToList()
                        select new
                        {
                            BookCategory = g.Key,

                            BookNames = string.Join(" ; ",list.Select(x => x.Field<string>("BookName").ToString()))
                        };