Oracle層次查詢（connect by ）是結(jié)構(gòu)化查詢中用到的，下面就為您介紹Oracle層次查詢的基本語(yǔ)法，希望可以讓您對(duì)Oracle層次查詢有更深的認(rèn)識(shí)。

oracle中的select語(yǔ)句可以用START WITH...CONNECT BY PRIOR子句實(shí)現(xiàn)遞歸查詢，connect by 是結(jié)構(gòu)化查詢中用到的，其基本語(yǔ)法是：

select ... from <TableName>
where <Conditional-1>
start with <Conditional-2>
connect by <Conditional-3>
;
<Conditional-1>：過(guò)濾條件，用于對(duì)返回的所有記錄進(jìn)行過(guò)濾。
<Conditional-2>：查詢結(jié)果重起始根結(jié)點(diǎn)的限定條件。
<Conditional-3>：連接條件

數(shù)據(jù)組織結(jié)構(gòu)如下圖：

數(shù)據(jù)庫(kù)表結(jié)構(gòu)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(shù)：
select * from t2 start with root_id = 0 connect by prior id = root_id;

獲取特定子樹(shù)：
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(jìn)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：

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