1、作用：
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。
2、语法
SELECT ...
FROM ....
START WITH cond1
CONNECT BY cond2
WHERE cond3;
2.1、说明
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
3、构造数据
-- 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');
4、查询jack下的所有子节点
select * from employee start with emp_name='jack' connect by prior emp_id=lead_id;emp_id | lead_id | emp_name | salary | dept_no
--------+---------+----------+--------+---------2 | 1 | jack | 50500 | 0024 | 2 | scott | 30000 | 0025 | 2 | tiger | 25000 | 002
(3 rows)
5、查询jack和arise下的所有子节点
select * from employee start with emp_name in ('jack', 'arise') connect by prior emp_id=lead_id;emp_id | lead_id | emp_name | salary | dept_no
--------+---------+----------+--------+---------2 | 1 | jack | 50500 | 0024 | 2 | scott | 30000 | 0025 | 2 | tiger | 25000 | 0023 | 1 | arise | 60000 | 0036 | 3 | wudde | 23000 | 0037 | 3 | joker | 21000 | 003
(6 rows)
6、查询jack的祖先节点
select * from employee start with emp_name='jack' connect by prior lead_id=emp_id;emp_id | lead_id | emp_name | salary | dept_no
--------+---------+----------+---------+---------2 | 1 | jack | 50500 | 0021 | 0 | king | 1000000 | 001
(2 rows)
7、查询一个节点的叔叔伯父节点
--查看emp_id为6的节点的叔叔伯父节点
with temp as (select employee.*,prior emp_name,level lefrom employee start with lead_id = 0connect by lead_id=prior emp_id
)
select *
from temp t
left join temp tt on tt.emp_id=6 --此处需要限定
where t.le = (tt.le-1)and t.emp_id not in (tt.lead_id);emp_id | lead_id | emp_name | salary | dept_no | ?column? | le | emp_id | lead_id | emp_name | salary | dept_no | ?column? | le
--------+---------+----------+--------+---------+----------+----+--------+---------+----------+--------+---------+----------+----2 | 1 | jack | 50500 | 002 | king | 2 | 6 | 3 | wudde | 23000 | 003 | arise | 3
(1 row)
8、查询族兄
--查看employee id是6的节点的族兄节点
with temp as (select employee.*,prior emp_name,level lefrom employee start with lead_id=0connect by lead_id= prior emp_id
)select t.*
from temp t
left outer join temp tton tt.emp_id=6 --此处需要条件限制
where t.le=tt.le and t.emp_id<>6; --此处需要条件限制 emp_id | lead_id | emp_name | salary | dept_no | ?column? | le
--------+---------+----------+--------+---------+----------+----4 | 2 | scott | 30000 | 002 | jack | 35 | 2 | tiger | 25000 | 002 | jack | 37 | 3 | joker | 21000 | 003 | arise | 3
(3 rows)
9、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name | emp_id | lead_id | salary | level
-------------+--------+---------+---------+-------king | 1 | 0 | 1000000 | 1jack | 2 | 1 | 50500 | 2scott | 4 | 2 | 30000 | 3tiger | 5 | 2 | 25000 | 3arise | 3 | 1 | 60000 | 2wudde | 6 | 3 | 23000 | 3joker | 7 | 3 | 21000 | 3
(7 rows)
10、connect_by_root 查找根节点
select connect_by_root emp_name,emp_name,lead_id,salary
from employee
start with lead_id=1
connect by prior emp_id = lead_id;connect_by_root | emp_name | lead_id | salary
-----------------+----------+---------+--------jack | jack | 1 | 50500jack | scott | 2 | 30000jack | tiger | 2 | 25000arise | arise | 1 | 60000arise | wudde | 3 | 23000arise | joker | 3 | 21000
(6 rows)
-- 注意： connect_by_root关键字后面跟着字段，表示根节点对应记录的某一字段的值，-- 如 connect_by_root emp_name表示根节点的员工名，connect_by_root salary表示根节点的工资
11、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;emp_id | emp_name | lead_id | salary | connect_by_isleaf
--------+----------+---------+---------+-------------------1 | king | 0 | 1000000 | 02 | jack | 1 | 50500 | 04 | scott | 2 | 30000 | 15 | tiger | 2 | 25000 | 13 | arise | 1 | 60000 | 06 | wudde | 3 | 23000 | 17 | joker | 3 | 21000 | 1
(7 rows)
-- 叶节点指的是没有子节点的节点，那些是既是父节点又是子节点的节点不属于叶节点
12、使用connect by rownum生成序列
ROWNUM是一个伪列，即先查到结果集之后再加上去的一个列，它的取值从1开始排依次递增。ROWNUM其实是oracle数据库从数据文件或缓冲区中读取数据的顺序。取得第一条记录则rownum值为1，第二条为2，依次类推。
connect by rownum是通过递归迭代第一行生成一个序列。格式如下：
select ***
from dual
connect by rownum<=n;
举例：
12.1 生成1-5之间的一个序列
select rownum
from dual
connect by rownum<=5;rownum
--------12345
(5 rows)
12.2 生成10个60到100之间的随机整数
select rownum No,ROUND(DBMS_RANDOM.VALUE(60,100),0) Value
from dual
connect by rownum<=10;no | value
----+-------1 | 772 | 643 | 674 | 865 | 826 | 777 | 948 | 899 | 6110 | 93
(10 rows)
12.3 生成连续的日期值
select rownum No,sysdate+rownum MyDate
from dual
connect by rownum<=10;no | mydate
----+---------------------1 | 2023-10-12 15:45:392 | 2023-10-13 15:45:393 | 2023-10-14 15:45:394 | 2023-10-15 15:45:395 | 2023-10-16 15:45:396 | 2023-10-17 15:45:397 | 2023-10-18 15:45:398 | 2023-10-19 15:45:399 | 2023-10-20 15:45:3910 | 2023-10-21 15:45:39
(10 rows)
