一句SQL实现MYSQL的递归查询

众所周知，目前的mysql版本中并不支持直接的递归查询，但是通过递归到迭代转化的思路，还是可以在一句SQL内实现树的递归查询的。这个得益于Mysql允许在SQL语句内使用@变量。以下是示例代码。

创建表格

CREATE TABLE `treenodes` (
    `id` int , -- 节点ID
    `nodename` varchar (60), -- 节点名称
    `pid` int  -- 节点父ID
); 

插入测试数据

INSERT INTO `treenodes` (`id`, `nodename`, `pid`) VALUES
('','A',''),('','B',''),('','C',''),
('','D',''),('','E',''),('','F',''),
('','G',''),('','H',''),('','I',''),
('','J',''),('','K',''),('','L',''),
('','M',''),('','N',''),('','O',''),
('','P',''),('','Q',''),('','R',''),
('','S',''),('','T',''),('','U','');

查询语句

 SELECT id AS ID,pid AS 父ID ,levels AS 父到子之间级数, paths AS 父到子路径 FROM (
     SELECT id,pid,
     @le:= IF (pid = 0 ,0,
         IF( LOCATE( CONCAT('|',pid,':'),@pathlevel)   > 0  ,
                  SUBSTRING_INDEX( SUBSTRING_INDEX(@pathlevel,CONCAT('|',pid,':'),-1),'|',1) +1
        ,@le+1) ) levels
     , @pathlevel:= CONCAT(@pathlevel,'|',id,':', @le ,'|') pathlevel
      , @pathnodes:= IF( pid =0,',0',
           CONCAT_WS(',',
           IF( LOCATE( CONCAT('|',pid,':'),@pathall) > 0  ,
               SUBSTRING_INDEX( SUBSTRING_INDEX(@pathall,CONCAT('|',pid,':'),-1),'|',1)
              ,@pathnodes ) ,pid  ) )paths
    ,@pathall:=CONCAT(@pathall,'|',id,':', @pathnodes ,'|') pathall
        FROM  treenodes,
    (SELECT @le:=0,@pathlevel:='', @pathall:='',@pathnodes:='') vv
    ORDER BY  pid,id
    ) src
ORDER BY id

最后的结果如下：

    ID   父ID  父到子之间级数  父到子路径
------  ------  ------------  ---------------
     1       0        0           ,0
     2       1        1           ,0,1
     3       1        1           ,0,1
     4       2        2           ,0,1,2
     5       2        2           ,0,1,2
     6       3        2           ,0,1,3
     7       6        3           ,0,1,3,6
     8       0        0           ,0
     9       8        1           ,0,8
    10       8        1           ,0,8
    11       8        1           ,0,8
    12       9        2           ,0,8,9
    13       9        2           ,0,8,9
    14      12        3           ,0,8,9,12
    15      12        3           ,0,8,9,12
    16      15        4           ,0,8,9,12,15
    17      15        4           ,0,8,9,12,15
    18       3        2           ,0,1,3
    19       2        2           ,0,1,2
    20       6        3           ,0,1,3,6
    21       8        1           ,0,8