Oracle start with...connect by prior用法（备忘）
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：
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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;
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????? 结构图：
???????????????? |---------a------a1
?????????????????|??????????? |
???????????????? |??????????? |
?????????????????|??????????? |------a2
???????????????? |
???????????????? |---------b------b1
?????????????????|??????????? |
???????????????? |??????????? |
?????????????????|??????????? |------b2
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2. 获取特定子树：
?2.1 select * from t2 start with id = 1 connect by prior id = root_id;
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?????? 结构图：
?????????????????|---------a------a1
?????????????????|??????????? |
???????????????? |??????????? |
?????????????????|??????????? |------a2
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?2.2 select * from t2 start with id = 4 connect by prior id = root_id;
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?????? 结构图：
???????????????? |---------b------b1
?????????????????|??????????? |
???????????????? |??????????? |
?????????????????|??????????? |------b2
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说明：如果connect by prior中的prior被省略，则查询将不进行深层递归。
例如：
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??????????select * from t2 start with root_id = 0 connect by id = root_id;
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???????????????? |---------a?????????????????
???????????????? |???????????
???????????????? |
???????????????? |---------b
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????????? select * from t2 start with id = 1 connect by id = root_id;
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???????????????? |---------a???
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以上内容从其他网友那里copy加工而成，特别感谢实例的原创者！！！
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