On TClientDataSets, that are not connected to a provider but loaded as local text files,
('thin client in brief-case mode'), you cannot use TQuerys.
You may use the .Filter property to select data and use an index to sort.
This sample code shows how to create a temporary index (not saved by SaveToFile())
and how to create a permanent index (saved by SaveToFile()):
With ClientDataSet1 Do
Begin
Close;
// Define the fields
FieldDefs.Clear;
FieldDefs.Add ('Project', ftInteger, 0, True);
FieldDefs.Add ('Number', ftInteger, 0, False);
// [..]
// Define the PERMANENT index - it is saved with SaveToFile()
IndexDefs.Clear;
IndexDefs.Add ('IProject', 'Project', [ixPrimary, ixUnique]);
// Create the dataset
CreateDataSet;
Open;
// the following temporary index is not saved
// with data when using SaveToFile()
AddIndex('TempIndex', 'Number;Project', [ixPrimary, ixUnique]);
End { with ClientDataSet1 }