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 SET STATISTICS IO ON;
 SET STATISTICS TIME ON;
 GO
 IF OBJECT_ID(N'dbo.t1') IS NOT NULL
     DROP TABLE dbo.t1;
 GO
 CREATE TABLE t1
     (
       id INT IDENTITY(1, 1)
              PRIMARY KEY ,
       a NVARCHAR(2) ,
       b NVARCHAR(2)
     );
 --建立索引a
 CREATE NONCLUSTERED INDEX [NonClusteredIndex-20190507-201219] ON [dbo].[t1]
 (
 [a] ASC
 )
 INCLUDE (     [b]) WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY];
 GO
 --建立索引b
 CREATE NONCLUSTERED INDEX [NonClusteredIndex-20190507-201235] ON [dbo].[t1]
 (
 [b] ASC
 )
 INCLUDE (     [a]) WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY];
 GO

 WITH    cte_0 ( a, b )
           AS ( SELECT   1 ,
                         2
                UNION ALL
                SELECT   1 ,
                         3
                UNION ALL
                SELECT   2 ,
                         4
                UNION ALL
                SELECT   2 ,
                         5
                UNION ALL
                SELECT   2 ,
                         3
                UNION ALL
                SELECT   3 ,
                         6
                UNION ALL
                SELECT   4 ,
                         9
                UNION ALL
                SELECT   5 ,
                         7
                UNION ALL
                SELECT   6 ,
                         9
                UNION ALL
                SELECT   7 ,
                         8
                UNION ALL
                SELECT   8 ,
                         10
                UNION ALL
                SELECT   9 ,
                         10
                UNION ALL
                SELECT   9 ,
                         12
                UNION ALL
                SELECT   10 ,
                         11
                UNION ALL
                SELECT   11 ,
                         12
              )
     INSERT  INTO t1
             ( a ,
               b
             )
             SELECT  a ,
                     b
             FROM    cte_0
             UNION ALL
             SELECT  b ,
                     a
             FROM    cte_0;
 --加上是否循环的标记（效率较不加循环标记要高）
 WITH    cte_a
           AS ( SELECT   a ,
                         b
                FROM     dbo.t1
                --UNION ALL
                --SELECT   b ,
                --         a
                --FROM     dbo.t1
              ),
         cte_b
           AS ( SELECT   a ,
                         b ,
                         0 AS isok ,
                         CAST(a + '>' + b AS NVARCHAR(MAX)) AS c_path ,
                         1 AS nlevel
                FROM     cte_a
                WHERE    a = ''--起点
                UNION ALL
                SELECT   s.a ,
                         s.b ,
                         CASE WHEN s.b = '' THEN 1--如果下一层找到了终点，那么不需要继续向下找
                              ELSE 0
                         END AS isok ,
                         CAST(p.c_path + '>' + s.b AS NVARCHAR(MAX)) AS c_path ,
                         p.nlevel + 1
                FROM     cte_a AS s
                         INNER JOIN cte_b AS p ON p.b = s.a
                WHERE    CHARINDEX(s.b, p.c_path) = 0
                         AND p.isok = 0
              ),
         cte_c
           AS ( SELECT   *
                FROM     cte_b
                --WHERE    b = '11'--终点
              )
     SELECT  *
     FROM    cte_c
     --WHERE   cte_c.nlevel = ( SELECT MIN(cte_c.nlevel)
     --                         FROM   cte_c
     --                       );
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