oracle进阶之connect by笔记

connect by 用于存在父子，祖孙，上下级等层级关系的数据表进行层级查询。

语法格式:
　　　　{ CONNECT BY [ NOCYCLE ] condition [AND condition]... [ START WITH condition ]
　　　　| START WITH condition CONNECT BY [ NOCYCLE ] condition [AND condition]...
　　　　}

特殊词讲解：

　　　　start with: 指定起始节点的条件

　　　　connect by: 指定父子行的条件关系

　　　　prior: 查询父行的限定符，格式: prior column1 = column2 or column1 = prior column2 and ... ，

　　　　nocycle: 若数据表中存在循环行，那么不添加此关键字会报错，添加关键字后，便不会报错，但循环的两行只会显示其中的第一条

　　　　循环行: 该行只有一个子行，而且子行又是该行的祖先行

　　　　connect_by_iscycle: 前置条件:在使用了nocycle之后才能使用此关键字，用于表示是否是循环行，0表示否，1 表示是

　　　　connect_by_isleaf: 是否是叶子节点，0表示否，1 表示是

　　　　level: level伪列,表示层级，值越小层级越高，level=1为层级最高节点

　　自定义数据:

-- 创建表
create table employee(
emp_id number(18),
lead_id number(18),
emp_name varchar2(200),
salary number(10,2),
dept_no varchar2(8)
);

-- 添加数据
insert into employee values('1',0,'king','1000000.00','001');
insert into employee values('2',1,'jack','50500.00','002');
insert into employee values('3',1,'arise','60000.00','003');
insert into employee values('4',2,'scott','30000.00','002');
insert into employee values('5',2,'tiger','25000.00','002');
insert into employee values('6',3,'wudde','23000.00','003');
insert into employee values('7',3,'joker','21000.00','003');commit;

数据列表展示:

数据树形展示:

(1) 查询以emp_id为0开始的节点的所有直属节点

select emp_id,lead_id,emp_name,prior emp_name as lead_name,salary
     from employee
     start with  lead_id=0
     connect by prior emp_id =  lead_id 

　　-- 等同于

　　　select emp_id,lead_id,emp_name,prior emp_name as lead_name,salary
     from employee
     start with  emp_id=1
     connect by prior emp_id =  lead_id

(2) 以emp_id为6的所有祖先节点

   select emp_id,lead_id,emp_name,salary
       from employee 
       start with emp_id=6
       connect by prior lead_id=emp_id;

(3) 查询一个节点的叔叔伯父节点

 with t as (
       select  employee.*,prior emp_name,level le
       from employee 
       start with lead_id = 0
       connect by lead_id=prior emp_id
       )
       select *
       from t 
       left join t tt on tt.emp_id=6
       where t.le = (tt.le-1)
       and t.emp_id not in (tt.lead_id)    

(4) 查询族兄

with t as (
            select employee.*,prior emp_name,level le
           from employee 
           start with lead_id=0
           connect by lead_id= prior emp_id
       )
       select t.*
       from t  t
       left join t tt on tt.emp_id=6
       where t.le=tt.le and t.emp_id<>6;

(5) level伪列的使用，格式化层级

  select lpad(' ',level*2,' ')||emp_name as name,emp_id,lead_id,salary,level
       from employee
       start with lead_id=0
       connect by prior emp_id=lead_id

level数值越低级别越高

(6) connect_by_root 查找根节点

    select connect_by_root emp_name,emp_name,lead_id,salary
       from employee  
       where dept_no='002'
       start with lead_id=1
       connect by  prior emp_id = lead_id;

(7) 标注循环行

  -- 插入一条数据，与另一条emp_id=7的数据组成循环行
       insert into employee values('3',7,'joker_cycle','21000.00','003');
       commit;
       -- connect_by_iscycle("CYCLE"), connect by nocycle
       select emp_id,emp_name,lead_id,salary,connect_by_iscycle as cycle 
       from employee 
       start with lead_id=0
       connect by nocycle prior emp_id = lead_id;

(8) connect_by_isleaf 是否是叶子节点

 select emp_id,emp_name,lead_id,salary,connect_by_isleaf
       from employee
       start with lead_id=0
       connect by nocycle prior emp_id=lead_id;

参照来源（知名博主）：https://www.cnblogs.com/king-xg/p/6794562.html