I'd like to ask what in your opinion is the best way to store trees in
SQL for use with XSL translation.

I think we can naturally imagine the relational database as an XML
tree, where we would see the tables, rows, and columns as elements in
the XML document.

For e.g. a list of employees (an example taken from Oracle RDBMS docs)
can be modeled as the XML structure:

<EMPLOYEES>
 <ROW>
   <EMPLOYEE_ID>205</EMPLOYEE_ID>
   <LAST_NAME>Higgins</LAST_NAME>
   <SALARY>12000</SALARY>
   ..<!-- other columns -->
  </ROW>
  ... <!-- other rows -->
</EMPLOYEES>

Regards,
Mukul