xml - Loop to get all Line details which have one Header

Hi all,

I have one item with Header and many line items.

I want to export to xml.

I need to use header as root in .xml because there is a possibility whereby no line item at all.

When i export out the data, only one header together with one line item can be exported.
I want to do loop to get all details.

I just need an idea. please help.