Its' a select statement directly from table with fields

Select doc.col1, doc.col2, doc.col3

from TestTable as doc

for xml path ('doc')

the result is one XML column or you can say it's one column, I didn't try your way but not working, see below:

SELECT ('<?xml version="1.0" encoding="iso8859-1"?>') for xml path ('')

UNION ALL

Select doc.col1, doc.col2, doc.col3

from TestTable as doc

for xml path ('doc')