当视图、内联函数、派生表和子查询包含集合运算符时，其中的 FOR XML 子句无效。若要解决这一问题，请使用派生表语法来包装内含集合运算符的 SELECT，并在其最上面应用 FOR XML。

The FOR XML clause is invalid in views, inline functions, derived tables, and subqueries when they contain a set operator. To work around, wrap the SELECT containing a set operator using derived table syntax and apply FOR XML on top of it.