Oracle Start with...Connect By子句

Start with...Connect By子句递归查询一般用于一个表维护树形结构的应用。
创建示例表：
CREATE TABLE TBL_TEST
(
  ID    NUMBER,
  NAME  VARCHAR2(100 BYTE),
  PID   NUMBER                                  DEFAULT 0
);
 
插入测试数据：
INSERT INTO TBL_TEST(ID,NAME,PID) VALUES('1','10','0');
INSERT INTO TBL_TEST(ID,NAME,PID) VALUES('2','11','1');
INSERT INTO TBL_TEST(ID,NAME,PID) VALUES('3','20','0');
INSERT INTO TBL_TEST(ID,NAME,PID) VALUES('4','12','1');
INSERT INTO TBL_TEST(ID,NAME,PID) VALUES('5','121','2');
 
从Root往树末梢递归
select * from TBL_TEST
 start with id=1
 connect by prior id = pid
 
从末梢往树ROOT递归
select * from TBL_TEST
 start with id=5
 connect by prior pid = id
=====
对于oracle进行简单树查询(递归查询)

DEPTID PAREDEPTID NAME
NUMBER NUMBER CHAR (40 Byte)
部门id 父部门id(所属部门id) 部门名称

通过子节点向根节点追朔.

Sql代码
select * from persons.dept start with deptid=76 connect by prior paredeptid=deptid   
Sql代码 
select * from persons.dept start with deptid=76 connect by prior paredeptid=deptid  
 select * from persons.dept start with deptid=76 connect by prior paredeptid=deptid 

通过根节点遍历子节点.

Sql代码
select * from persons.dept start with paredeptid=0 connect by prior deptid=paredeptid   
Sql代码 
select * from persons.dept start with paredeptid=0 connect by prior deptid=paredeptid  
select * from persons.dept start with paredeptid=0 connect by prior deptid=paredeptid 

可通过level 关键字查询所在层次.

Sql代码
select a.*,level from persons.dept a start with paredeptid=0 connect by prior deptid=paredeptid   
Sql代码 
select a.*,level from persons.dept a start with paredeptid=0 connect by prior deptid=paredeptid  
select a.*,level from persons.dept a start with paredeptid=0 connect by prior deptid=paredeptid 

再次复习一下:start with ...connect by 的用法， start with 后面所跟的就是就是递归的种子。

递归的种子也就是递归开始的地方 connect by 后面的"prior" 如果缺省：则只能查询到符合条件的起始行，并不进行递归查询；

connect by prior 后面所放的字段是有关系的，它指明了查询的方向。

练习： 通过子节点获得顶节点

Sql代码
select FIRST_VALUE(deptid) OVER (ORDER BY LEVEL DESC ROWS UNBOUNDED PRECEDING) AS firstdeptid from persons.dept start with deptid=76 connect by prior paredeptid=deptid