LINQ join on several tables in a dataset with a group by on several variables

Dear Experts,

I would want to join the table in a dataset ds and then do a group by on several variables. the below is the code i have attempted.

Antony

        Dim ct As DataTable = ds.Tables("ctime")
        Dim phh As DataTable = ds.Tables("perhour")
        Dim area As DataTable = ds.Tables("areas")
        Dim bs As DataTable = ds.Tables("station")

   Dim query = From hh In phh.AsEnumerable() Join t In ct.AsEnumerable() _
                    On t.Field(Of Integer)("time_key") Equals hh.Field(Of Integer)("time_key") _
                    Join ar In area.AsEnumerable() _
                    On hh.Field(Of Integer)("wallet_cd") Equals ar.Field(Of Integer)("wallet_cd") _
                    Join b In bs.AsEnumerable() _
                    On hh.Field(Of Integer)("base_station_key") Equals b.Field(Of Integer)("base_station_key") _
                    Group hh By customerid = hh.Field(Of Integer)("msisdn") Into grp = Group _
                       Select New With _
                            { _
                          Key customerid, _
                         .rev_dat_tot_nbn = grp.Sum(Function(x) If(x.Field(Of String)("usage_type_key") = 44, x.Field(Of Decimal)("call_cost_amt"), 0)),
                         .rev_voi_tot_nbn = grp.Sum(Function(x) If(x.Field(Of String)("usage_type_key") = 41, x.Field(Of Decimal)("call_cost_amt"), 0)),
                         .rev_sms_tot_nbn = grp.Sum(Function(x) If(x.Field(Of String)("usage_type_key") = 42, x.Field(Of Decimal)("call_cost_amt"), 0)),
                         .rev_tot_tot_nbn = grp.Sum(Function(x) x.Field(Of Decimal)("call_cost_amt")) _
                            }