xmlforest converts each of its argument parameters to XML,
and then returns an XML fragment that is the concatenation of these
converted arguments.
- If value_expr is a scalar expression, then you can omit
the AS clause, and Oracle uses the column name as the element name.
- If value_expr is an object type or collection, then the
AS clause is mandatory, and Oracle uses the specified c_alias as the
enclosing tag.
- If value_expr is null, then no element is created for
that value_expr.