作用
connect by主要用于父子，祖孙，上下级等层级关系的查询
语法
{ CONNECT BY [ NOCYCLE ] condition [AND condition]… [ START WITH condition ]
| START WITH condition CONNECT BY [ NOCYCLE ] condition [AND condition]…}
解释：
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
例子
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;
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
查询以emp_id为6开始的节点
select emp_id,lead_id,emp_name,salary
from employee
start with emp_id=6
connect by prior lead_id=emp_id;
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
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;
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;
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;
