https://www.cnblogs.com/always-online/archive/2015/10/30/4923532.html
oracle树形查询 start with connect by
一、简介
在oracle中start with connect by (prior) 用来对树形结构的数据进行查询。其中start with conditon 给出的是数据搜索范围,connect by后面给出了递归查询的条件,prior 关键字表示父数据，prior 条件表示子数据需要满足父数据的什么条件。如下
start with id= '10001' connect by prior parent_id= id and prior num = 5
表示查询id为10001，并且递归查询parent_id=id,为5的记录。
二、实例
1、构造数据
-- 表结构 create table menu( id varchar2(64) not null,parent_id varchar2(64) not null,name varchar2(100) not null,depth number(2) not null,primary key (id) ) -- 初始化数据 -- 顶级菜单 insert into menu values ('100000','0','顶级菜单1',1); insert into menu values ('200000','顶级菜单2',1); insert into menu values ('300000','顶级菜单3',1); -- 父级菜单 -- 顶级菜单1 直接子菜单 insert into menu values ('110000','100000','菜单11',2); insert into menu values ('120000','菜单12',2); insert into menu values ('130000','菜单13',2); insert into menu values ('140000','菜单14',2); -- 顶级菜单2 直接子菜单 insert into menu values ('210000','200000','菜单21',2); insert into menu values ('220000','菜单22',2); insert into menu values ('230000','菜单23',2); -- 顶级菜单3 直接子菜单 insert into menu values ('310000','300000','菜单31',2); -- 菜单13 直接子菜单 insert into menu values ('131000','130000','菜单131',3); insert into menu values ('132000','菜单132',3); insert into menu values ('133000','菜单133',3); -- 菜单132 直接子菜单 insert into menu values ('132100','132000','菜单1321',4); insert into menu values ('132200','菜单1332',4);
生成的菜单层次结构如下：
顶级菜单1
菜单11
菜单12
菜单13
菜单131
菜单132
菜单1321
菜单1322
菜单133
菜单14
顶级菜单2
菜单21
菜单22
菜单23
顶级菜单3
菜单31
2、SQL查询
--prior放的左右位置决定了检索是自底向上还是自顶向下. 左边是自上而下（找子节点），右边是自下而上（找父节点） --找父节点 select * from menu start with id='130000' connect by id = prior parent_id;
--找子节点节点 -- (子节点)id为130000的菜单，以及130000菜单下的所有直接或间接子菜单(prior 在左边， prior、parent_id(等号右边)在右边） select * from menu start with id='130000' connect by prior id = parent_id ;
-- (父节点)id为1321的菜单，以及1321菜单下的所有直接或间接父菜单(prior、parent_id(等号左边) 都在左边) select * from menu start with id='132100' connect by prior parent_id = id; -- prior 后面跟的是(parent_id) 则是查找父节点,prior后面跟的是(id)则是查找子节点
--根据菜单组分类统计每个菜单包含子菜单的个数 select id,max(name) name,count(1) from menu group by id connect by prior parent_id = id order by id
-- 查询所有的叶子节点 select t2.* from menu t2 where id not in(select t.parent_id from menu t) order by id;
三、性能问题
对于start with connect by语句的执行，oracle会进行递归查询，当数据量大的时候会产生性能相关问题。
--生成执行计划 explain plan for select * from menu start with id='132100' connect by prior parent_id = id; -- 查询执行计划 select * from table( dbms_xplan.display);
语句执行计划结果如下：
Plan hash value: 3563250490 ---------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%cpu)| Time | ---------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 133 | 1 (0)| 00:00:01 | |* 1 | CONNECT BY WITH FILTERING | | | | | | | 2 | TABLE ACCESS BY INDEX ROWID | MENU | 1 | 133 | 1 (0)| 00:00:01 | |* 3 | INDEX UNIQUE SCAN | SYS_C0018586 | 1 | | 1 (0)| 00:00:01 | | 4 | NESTED LOOPS | | | | | | | 5 | CONNECT BY PUMP | | | | | | | 6 | TABLE ACCESS BY INDEX ROWID| MENU | 1 | 133 | 1 (0)| 00:00:01 | |* 7 | INDEX UNIQUE SCAN | SYS_C0018586 | 1 | | 1 (0)| 00:00:01 | ---------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - access("ID"=PRIOR "PARENT_ID") 3 - access("ID"='132100') 7 - access("ID"=PRIOR "PARENT_ID") Note ----- - dynamic sampling used for this statement
通过该执行计划得知，改语句执行了7步操作，才将结果集查询并返回。当需要查询条件进行过滤的时候，我们可以通过查看执行计划从而对sql进行优化。
