Oracle Connect By Prior（递归查询）简洁实例

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>
;

<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

例如：

数据库表结构如下：

create table t2(
root_id number,
id number,
name varchar(5),
description varchar(10)
);

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

如图：

1.获取完整树：

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

2.获取特定子树：
select * from t2 start with id = 1 connect by prior id = root_id;

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

3.如果connect by prior中的prior被省略，则查询将不进行深层递归。
如：
select * from t2 start with root_id = 0 connect by id = root_id;

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

总结：

1、start with id= 是定义起始节点（种子），可以是id也可以是root_id,定义为root_Id查询该节点下所有的树结构，定义为id（子节点）则查询指定的树

2、connect by prior ：prior的含义为先前，前一条记录。prior id=root_id 也就是前一条记录

的id等于当前记录的root_id(父id)
3、可以向下或者向上查找

4、level字段为oracle特有的层级字段，可以通过level字段查询指定的层级

select root_id,level from t2 where level=1

start with root_id = 0

connect by prior id = root_id;

原文链接：https://www.f2er.com/oracle/208157.html

Oracle Connect By Prior（递归查询）简洁实例

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