作用

connect by主要用於父子，祖孫，上下級等層級關係的查詢

語法

{ CONNECT BY [ NOCYCLE ] condition [AND condition]... [ START WITH condition ]
| START WITH condition CONNECT BY [ NOCYCLE ] condition [AND condition]...}

解釋：

start with: 指定起始節點的條件

connect by: 指定父子行的條件關係

prior: 查詢父行的限定符，格式: prior column1 = column2 or column1 = prior column2 and ... ，

nocycle: 若資料表中存在迴圈行，那麼不新增此關鍵字會報錯，新增關鍵字後，便不會報錯，但迴圈的兩行只會顯示其中的第一條

迴圈行: 該行只有一個子行，而且子行又是該行的祖先行

connect_by_iscycle: 前置條件:在使用了nocycle之後才能使用此關鍵字，用於表示是否是迴圈行，0表示否，1 表示是

connect_by_isleaf: 是否是葉子節點，0表示否，1 表示是

level: level偽列,表示層級，值越小層級越高，level=1為層級最高節點

例子

自定義資料

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

-- 新增資料
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;

查詢以emp_id為0開始的節點的所有直屬節點

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 

　　-- 等同於

　　　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

查詢以emp_id為6開始的節點

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

level偽列的使用，格式化層級

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

connect_by_root 查詢根節點

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;

標註迴圈行

-- 插入一條資料，與另一條emp_id=7的資料組成迴圈行
       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;

connect_by_isleaf 是否是葉子節點

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;

轉自：https://www.cnblogs.com/wanggang-java/p/10916426.html