Oracle Connect By Function
Version 12.1.0.1

General Information
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The Library is currently in the process of being upgraded from Oracle Database Version 11.2.0.3 to 12.1.0.1. Demos are being upgraded to reflect the new Container paradigm as well as EBR (Edition Based Redefinition) and may contain references to CDBs, PDBs, and other objects you may not be familiar with such as CDB_OBJECTS_AE: Welcome to 12c.
Syntax 1 CONNECT BY [NOCYCLE] <condition> START WITH <condition>
Syntax 2 START WITH <condition> CONNECT BY [NOCYCLE] <condition>
 
CONNECT BY LEVEL
A condition that identifies the relationship between parent rows and child rows of the hierarchy CONNECT BY <child_value> = <parent_value>
conn uwclass/uwclass

CREATE TABLE t (
rid NUMBER(12),
col1 VARCHAR2(30),
col2 VARCHAR2(300));

CREATE SEQUENCE seq_t_rid;

INSERT INTO t
SELECT seq_t_rid.NEXTVAL, dbms_crypto.randombytes(15), dbms_crypto.randombytes(150)
FROM dual
CONNECT BY LEVEL <= 1000;
 
CONNECT BY PRIOR
A condition that identifies the relationship between parent rows and child rows of the hierarchy CONNECT BY <child_value> = <parent_value>
conn hr/hr

SELECT employee_id, last_name, manager_id
FROM employees
CONNECT BY PRIOR employee_id = manager_id;
 
START WITH
Specifies a condition that identifies the row(s) to be used as the root(s) of a hierarchical query START WITH (column_name) = <value>
conn hr/hr

SELECT last_name, employee_id, manager_id, LEVEL
FROM employees
START WITH employee_id = 100
CONNECT BY PRIOR employee_id = manager_id;

SELECT last_name, employee_id, manager_id, LEVEL
FROM employees
START WITH employee_id = 101
CONNECT BY PRIOR employee_id = manager_id;

set pagesize 0
col last_name format a30

SELECT LEVEL, LPAD(' ', LEVEL*3) || LAST_NAME AS LAST_NAME
FROM employees
START WITH employee_id = 100
CONNECT BY PRIOR employee_id = manager_id;
 
ORDER SIBLINGS BY
SIBLINGS BY preserves any ordering specified in the hierarchical query clause and then applies the order_by_clause to the siblings of the hierarchy ORDER SIBLINGS BY (column_name);
conn hr/hr

SELECT last_name, employee_id, manager_id, LEVEL
FROM employees
START WITH employee_id = 100
CONNECT BY PRIOR employee_id = manager_id
ORDER SIBLINGS BY last_name;
 
CONNECT_BY_ROOT
CONNECT_BY_ROOT is a unary operator that is valid only in hierarchical queries. When you qualify a column with this operator, Oracle returns the column value using data from the root row.

Cannot be specified with the START WITH or  CONNECT BY condition.
The following example returns the last name of each employee in department 110, each manager above that employee in the hierarchy, the number of levels between manager and employee, and the path between the two.
conn hr/hr

set linesize 121
col emp format a20
col mgr format a20

SELECT last_name
FROM employees
WHERE department_id = 110;

-- incorrect use (from Oracle docs)
SELECT last_name, CONNECT_BY_ROOT last_name MGR,
LEVEL-1 PATHLEN, SYS_CONNECT_BY_PATH(last_name, '/') PATH
FROM employees
WHERE LEVEL > 1
AND department_id = 110
CONNECT BY PRIOR employee_id = manager_id
ORDER BY last_name, MGR, PATHLEN, PATH;

-- correct use
SELECT last_name, CONNECT_BY_ROOT last_name,
LEVEL, SYS_CONNECT_BY_PATH(last_name, '/') "Path"
FROM employees
WHERE LEVEL <= 3 AND department_id = 80
START WITH last_name = 'King'
CONNECT BY NOCYCLE PRIOR employee_id = manager_id
AND LEVEL <= 4;
 
CONNECT_BY_ISCYCLE Pseudocolumn
The CONNECT_BY_ISCYCLE pseudocolumn returns 1 if the current row has a child which is also its ancestor. Otherwise returns 0. conn hr/hr

UPDATE employees SET manager_id = 145
WHERE employee_id = 100;

set linesize 121
col path format a50

-- incorrect use
SELECT last_name, LEVEL, SYS_CONNECT_BY_PATH(last_name, '/') "Path"
FROM employees
WHERE LEVEL <= 3 AND department_id = 80
START WITH last_name = 'King'
CONNECT BY PRIOR employee_id = manager_id AND LEVEL <= 4;
ERROR:
ORA-01436: CONNECT BY loop in user data

-- correct use
SELECT last_name, CONNECT_BY_ISCYCLE "Cycle", LEVEL, SYS_CONNECT_BY_PATH(last_name, '/') "Path"
FROM employees
WHERE LEVEL <= 3 AND department_id = 80
START WITH last_name = 'King'
CONNECT BY NOCYCLE PRIOR employee_id = manager_id
AND LEVEL <= 4;

ROLLBACK;
 
CONNECT_BY_ISLEAF Pseudocolumn
The CONNECT_BY_ISLEAF pseudocolumn returns 1 if the current row is a leaf of the tree defined by the CONNECT BY condition. Otherwise returns 0. This information indicates whether a given row can be further expanded to show more of the hierarchy. conn hr/hr

-- incorrect use
SELECT last_name "Employee", CONNECT_BY_ISLEAF "IsLeaf",
LEVEL, SYS_CONNECT_BY_PATH(last_name, '/') "Path"
FROM employees
WHERE level <= 3
AND department_id = 80
START WITH last_name = 'King'
CONNECT BY PRIOR employee_id = manager_id
AND LEVEL <= 4;

-- correct use
SELECT last_name "Employee", CONNECT_BY_ISLEAF "IsLeaf",
LEVEL, SYS_CONNECT_BY_PATH(last_name, '/') "Path"
FROM employees
WHERE level <= 3 AND department_id = 80
START WITH last_name = 'King'
CONNECT BY NOCYCLE PRIOR employee_id = manager_id
AND LEVEL <= 4;
 
LEVEL Pseudocolumn
For each row returned by a hierarchical query, the LEVEL pseudocolumn returns 1 for a root row, 2 for a child of a root, and so on LEVEL RETURN NUMBER
conn hr/hr

SELECT employee_id, last_name, manager_id, LEVEL
FROM employees
CONNECT BY PRIOR employee_id = manager_id;

SELECT LPAD(' ',2*(LEVEL-1)) || last_name ORG_CHART,
employee_id, manager_id, job_id
FROM employees
START WITH job_id = 'AD_VP'
CONNECT BY PRIOR employee_id = manager_id;
 
SYS_CONNECT_BY_PATH
Returns the path of a column value from root to node, with column values separated by char for each row returned by CONNECT BY condition SYS_CONNECT_BY_PATH(<column>, <char>)
conn scott/tiger

col empname format a20
col cbp format a30

SELECT LPAD(' ', 2*LEVEL, ' ' ) || ename empName, dname, job,
sys_connect_by_path( ename, '/' ) cbp
FROM emp e, dept d
WHERE e.deptno = d.deptno
START WITH mgr IS NULL
CONNECT BY PRIOR empno = mgr
ORDER SIBLINGS BY job;

SELECT LPAD(' ', 2*LEVEL, ' ' ) || ename empName, dname, job,
sys_connect_by_path(empno, '.') cbp
FROM scott.emp emp, scott.dept dept
WHERE emp.deptno = dept.deptno
START WITH mgr IS NULL
CONNECT BY PRIOR empno = mgr
ORDER SIBLINGS BY ename;
 
Function Demo
Use A Function To Receive The Current Node and Search for Parents of the Current Node CREATE OR REPLACE FUNCTION permissions_sub_tree_root (
the_id IN NUMBER,
the_level IN NUMBER)
RETURN NUMBER AUTHID DEFINER IS
 sub_tree_root NUMBER(10);
BEGIN
  SELECT id
  INTO sub_tree_root
  FROM hierarchy
  WHERE level = the_level
  -- Connect 'upwards', i.e. find the parent
  CONNECT BY PRIOR PARENT = id
  START WITH ID = the_id;

  RETURN sub_tree_root;
END permissions_sub_tree_root;
/

SELECT id, name, username
FROM (
  SELECT ID, PARENT, NAME,
  permissions_sub_tree_root
(id, LEVEL) ROOT
  FROM hierarchy
  CONNECT BY PRIOR id = PARENT) HIERARCHY, permissions
WHERE ROOT = hierarchy_id;
 
GROUP BY Demo
Group By Demo with CONNECT_BY_ROOT and CONNECT_BY_PRIOR conn hr/hr

SELECT name, SUM(salary) "Total_Salary"
FROM (
  SELECT CONNECT_BY_ROOT last_name name, salary
  FROM employees
  WHERE department_id = 110
  CONNECT BY NOCYCLE PRIOR employee_id = manager_id)
GROUP BY name;
 
Demos
Indenting conn hr/hr

col lname format a30

SELECT LPAD(' ', LEVEL*2, ' ') || last_name LNAME, d.department_id
FROM employees e, departments d
WHERE e.department_id = d.department_id
START WITH employee_id = 100
CONNECT BY PRIOR e.employee_id = e.manager_id;
Hierarchical Query with IN In a [NOT] IN condition in a WHERE clause, if the right-hand side of the condition is a subquery, you cannot use LEVEL on the left-hand side of the condition. However, you can specify LEVEL in a subquery of the FROM clause to achieve the same result. For example, the following statement is not valid:
-- this will fail
SELECT employee_id, last_name FROM employees
WHERE (employee_id, LEVEL)
IN (SELECT employee_id, 2 FROM employees)
START WITH employee_id = 2
CONNECT BY PRIOR employee_id = manager_id;
But the following statement is valid because it encapsulates the query containing the LEVEL information in the FROM clause:
-- this will succeed
SELECT v.employee_id, v.last_name, v.lev
FROM (
  SELECT employee_id, last_name, LEVEL lev
  FROM employees v
  START WITH employee_id = 100
  CONNECT BY NOCYCLE PRIOR employee_id = manager_id) v
WHERE (v.employee_id, v.lev) IN (
  SELECT employee_id, 2 FROM employees);

Related Topics
Common Table Expressions
Oracle Built-in Functions
SELECT Statements

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