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      

以上就是一句SQL实现MYSQL的递归查询的实现全过程，希望对大家的学习有所帮助 。