var ds = new DataSet();
ds.ReadXml("items.xml");
var query = from detail in ds.Tables[1].AsEnumerable()
            join master in ds.Tables[0].AsEnumerable() on detail["master_Id"] equals master["master_Id"]
            where detail["title"].ToString() == "x"
            orderby master["title"]
            select new
            {
                master,
                delimeterstring = ds.Tables[1].AsEnumerable().Where(d => (int)d["master_Id"] == (int)master["master_Id"]).Select(d => (string)d["title"]).Aggregate((a, b) => string.Format("{0},{1}", a, b)),
                childcount = ds.Tables[1].AsEnumerable().Count(d => (int)d["master_Id"] == (int)master["master_Id"])
            };