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。