Forms :: Datasheet Combo Box Population By Query

The datasheet is populated in a subform, based upon a table.

Code:
SELECT DEV_List.*
FROM DEV_List;

Within this datasheet, I have several combo boxes.It is easy to give them a query to show all values possible.

Code:
SELECT tbl_PPV_RPM.Createdby
FROM tbl_PPV_RPM
GROUP BY tbl_PPV_RPM.Createdby;

This gives me a list of all possible values in "tbl_PPV_RPM.Createdby".What I want, is to be able to select only the relevant values in this combobox, based upon a materialnumber in the same row. "tbl_PPV_RPM" contains a row named "Material". The datasheet also contains a row named "Material". These need to be linked. When I try to link them through the query builder, I get this:

Code:
SELECT tbl_PPV_RPM.Createdby
FROM tbl_PPV_RPM
WHERE (((tbl_PPV_RPM.Material)=[Forms]![frmSub_TD_List_Edit].[Material]))
GROUP BY tbl_PPV_RPM.Createdby;

But then I get a popup box, requesting for the Material number in "frmSub_TD_List_Edit".So, it does not recognise the Materialnumber in the row I'm trying to select a value.