Oracle層次查詢（connect by ）是結構化查詢中用到的，下面就為您介紹Oracle層次查詢的基本語法，希望可以讓您對Oracle層次查詢有更深的認識。
Oracle中的select語句可以用START WITH...CONNECT BY PRIOR子句實現遞歸查詢，connect by 是結構化查詢中用到的，其基本語法是：
select ... from <TableName>
where <Conditional-1>
start with <Conditional-2>
connect by <Conditional-3>
;
<Conditional-1>：過濾條件，用於對返回的所有記錄進行過濾。
<Conditional-2>：查詢結果重起始根結點的限定條件。
<Conditional-3>：連接條件
數據組織結構如下圖：
數據庫表結構如下：
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');
獲取完整樹：
select * from t2 start with root_id = 0 connect by prior id = root_id;
獲取特定子樹：
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，則查詢將不進行深層遞歸。
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：
