SQL如何实现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
('1','A','0'),('2','B','1'),('3','C','1'),
('4','D','2'),('5','E','2'),('6','F','3'),
('7','G','6'),('8','H','0'),('9','I','8'),
('10','J','8'),('11','K','8'),('12','L','9'),
('13','M','9'),('14','N','12'),('15','O','12'),
('16','P','15'),('17','Q','15'),('18','R','3'),
('19','S','2'),('20','T','6'),('21','U','8');

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