Oracle层次查询（connect by ）是结构化查询中用到的，下面就为您介绍Oracle层次查询的基本语法，希望可以让您对Oracle层次查询有更深的认识。
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>：连接条件数据组织结构如下图：
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,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');获取完整树：select * from t2 start with root_id = 0 connect by prior id = root_id;
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;
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;如：
