Question

I have a Datatable in my VB code that I want to group in two levels (In other words group within another group) and I’m having a problem doing it.

For illustration’s sake, suppose my datatable, MyTable, had columns as follows:

Name1     Name2    Start_Date    End_Date   Var1    Var2

So, supposing I wanted to just group it by, say, Name1, I could do it as follows:

Dim Query1 = From dr as DataRow in MyTable
             Group dr by Name1 = dr.Item("Name1") into Group

And then I could loop through all the records in that group doing something along the lines of:

For Each Grp in Query1
   For Each dr as DataRow in Grp.Rows
      ... do whatever ...
   Next
Next

Now, I would like to create a double group – For Example, suppose I wanted to create an in-memory query that first groups by Start_Date and End_Date and then, WITHIN THAT GROUP, a second grouping by Name1 so I could write a loop along the lines of:

For Each MainGrp in BiggerQuery
   For Each SubGrp in MainGrp
        For Each dr as DataRow in SubGrp .Rows
            ... do whatever ...
        Next
   Next
Next

How would that double-grouped Linq query look in VB? – My first issue is that it seems that VB requires you to name your group Group, which doesn’t allow for double-grouping and secondly, I can’t seem to figure out the query as a whole.

Thanks!!!

Answer 1

Dim query = From dr1 As DataRow In MyTable
            Group dr1 By New With { .StartDate = dr1.StartDate, .EndDate = dr1.EndDate } Into g1 = Group
            Select newGroup = (From dr2 As DataRow In g1
                    Group dr2 By dr2.Name1 Into g2 = Group
                    Select g2)

I think it should work, but cannot test it right now.

EDIT

There is a nice example of nested grouping on MSDN: http://msdn.microsoft.com/en-us/vstudio/bb737908#grpbynest

EDIT2

From dr1 As DataRow In myTable
Group dr1 By gi1 = New With {.StartDate = dr1.Item("StartDate"), .EndDate = dr1.Item("EndDate")} Into g1 = Group
Select newGroup = (From dr2 As DataRow In g1
                   Group dr2 By gi2 = dr2.Item("Name1") Into g2 = Group
                   Select g2)