查询树形的根节点

数据库环境：SQL SERVER 2005

　　有一个test表，其表结构及数据如下图1。其中，id是主键，mid是当前节点，pid是父节点。

要求：查出每个节点的根节点，如图2所示。

查询树形的根节点

　　分析：这需求实际上树形查询的扩展，我们可以先找到根节点，从根节点往下找到分支节点，

再从分支节点往下找叶子节点。

　　1.数据准备

WITH    x0
          AS ( SELECT   1 AS id ,
                        'A' AS mid ,
                        'B' AS pid
               UNION ALL
               SELECT   2 AS id ,
                        'B' AS mid ,
                        'C' AS pid
               UNION ALL
               SELECT   3 AS id ,
                        'C' AS mid ,
                        'N' AS pid
               UNION ALL
               SELECT   4 AS id ,
                        'D' AS mid ,
                        'E' AS pid
               UNION ALL
               SELECT   5 AS id ,
                        'E' AS mid ,
                        'G' AS pid
               UNION ALL
               SELECT   6 AS id ,
                        'G' AS mid ,
                        'K' AS pid
               UNION ALL
               SELECT   7 AS id ,
                        'J' AS mid ,
                        'H' AS pid
             )

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