如何在父标记数据为空时添加子标记，Union all用于构建XML

Question

我有以下情况。我使用Union all来构建XML。第一个查询的结果被视为第二个select语句的父标记。我的查询如下所示。对此有什么解决方法？

SELECT '<MasterRecordsXML>'

SELECT distinct
     1                                      AS TAG
    ,NULL                                   AS parent
    ,d              AS [item!1!id]
    ,c              AS [item!1!dsc]
from Table1

Union all

SELECT distinct
     2                                      AS TAG
    ,1                                  AS parent
    ,d              AS [item!1!id]
    ,c              AS [item!1!dsc]
from Table2