Oracle中的select语句可以用start with...connect by prior子句实现递归查询，connect by 是结构化查询中用到的，其基本语法是：

??????? select ...

????????? from <TableName>?
?????? where <Conditional-1>
? start with <Conditional-2>
connect by <Conditional-3>;

说明：
<Conditional-1>：过滤条件，用于对返回的所有记录进行过滤。
<Conditional-2>：查询结果重起始根结点的限定条件。
<Conditional-3>：连接条件

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还是看个例子吧，应该能更好的说明问题。数据库表结构如下：

create table t2(
???????root_id? number,
??????????????? id number,
????????? name?varchar(5),

? description varchar(10)
);

插入测试数据：

?

insert into t2(root_id,id,name,description) values(0,1,'a','aaa');
insert into t2(root_id,id,name,description) values(1,2,'a1','aaa1');
insert into t2(root_id,id,name,description) values(1,3,'a2','aaa2');
insert into t2(root_id,id,name,description) values(0,4,'b','bbb');
insert into t2(root_id,id,name,description) values(4,5,'b1','bbb1');
insert into t2(root_id,id,name,description) values(4,6,'b2','bbb2');

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---------------------------------------------------------------

--------------------------------------准备好了，开始吧-----------------------------------------

---------------------------------------------------------------

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1. 获取完整树：

????? select * from t2 start with root_id = 0 connect by prior id = root_id;

?

????? 结构图：

???????????????? |---------a------a1

?????????????????|??????????? |

???????????????? |??????????? |

?????????????????|??????????? |------a2

???????????????? |

???????????????? |---------b------b1

?????????????????|??????????? |

???????????????? |??????????? |

?????????????????|??????????? |------b2

2. 获取特定子树：

?2.1 select * from t2 start with id = 1 connect by prior id = root_id;

?

?????? 结构图：

?????????????????|---------a------a1

?????????????????|??????????? |

???????????????? |??????????? |

?????????????????|??????????? |------a2

?

?2.2 select * from t2 start with id = 4 connect by prior id = root_id;

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?????? 结构图：

???????????????? |---------b------b1

?????????????????|??????????? |