How to traverse Hierarchical data in Oracle and sort the rows at the same level in a hierarchy?

Problem Statement: You need to traverse the hierarchy data from top to bottom and sort the rows at the same level in a hierarchy.

Solution: The usual ORDER BY clause will not sort the rows at the same hierarchy level. We need to use SIBLINGS with in the ORDER BY Clause.

Additionally, Oracle provides CONNECT BY clause to specify a hierarchical query i.e. how to connect the parent nodes and child nodes and the PRIOR operator to define the join condition/s between the parent nodes, and the LEVEL pseudo-column to indicate how far from the root/parent row the current row is.

Additionally, we can use the START WITH clause to indicate where to start the tree navigation. We must use the PRIOR operator to specify the column/s in the parent row that have the same values as the linking columns in the child row.

In order to loop through the entire hieratchy, remove the START WITH clause. This will fetch all the students with their professor and their professor’s professor(i.e. Dean), and so on, until the top manager/director of the institute.

Example 1: Sort the rows at the same level in a hierarchy with in a Parent row for the professor_id = 102 SQL:

/*
     Function - Identify the hierarchical/structure for the professor_id = 102
  Tables Used - students
         Data - documented below
*/

SELECT student_id,
       level,
       lpad('-',(level-1)*3) || first_name || ' ' || last_name AS full_name
  FROM students
START WITH professor_id  = 102
CONNECT BY professor_id  = PRIOR student_id
ORDER SIBLINGS by first_name, last_name;

Output

103 1   BROWN MICHAEL
106 2     -DAVIS RICHARD
107 2     -GARCIA CHARLES
104 2     -JONES WILLIAM
105 2     -MILLER DAVID

Example 2: Sort the rows at the same level in a hierarchy with in a Parent row where the professor_id is null SQL:

/*
     Function - Sort the rows at the same level in a hierarchy with in a Parent row
  Tables Used - students
         Data - documented below
*/

SELECT student_id,
       level,
       lpad('-',(level-1)*3) || first_name || ' ' || last_name AS full_name
  FROM students
START WITH professor_id IS null
CONNECT BY professor_id = PRIOR student_id
ORDER SIBLINGS by first_name, last_name;

Output

100 1   SMITH JAMES
101 2     -JOHNSON JOHN
108 3        -RODRIGUEZ JOSEPH
111 4           -ANDERSON DANIEL
110 4           -MARTINEZ CHRISTOPHER
112 4           -TAYLOR PAUL
113 4           -THOMAS MARK
109 4           -WILSON THOMAS
102 2     -WILLIAMS ROBERT
103 3        -BROWN MICHAEL
106 4           -DAVIS RICHARD
107 4           -GARCIA CHARLES
104 4           -JONES WILLIAM
105 4           -MILLER DAVID

The sorting obviously occurs at each level of the hierarchy. The output shows under SMITH JAMES, the direct subordinates are JOHNSON JOHN and WILLIAMS ROBERT in alphabetical order within level 2. Under JOHNSON JOHN we have further subordinates at level 3 fully sorted in alphanetical order.

Data Preparation: Data used for the problem is shown below. The data is completely made up for demonstration purpose.

Example

DROP TABLE students;
COMMIT;

CREATE TABLE students
    ( student_id     NUMBER(6)
    , first_name     VARCHAR2(20)
    , last_name      VARCHAR2(25) 
    , email          VARCHAR2(40) 
    , phone_number   VARCHAR2(20)
    , join_date      DATE 
    , class_id       VARCHAR2(20)   
    , fees           NUMBER(8,2)
    , professor_id   NUMBER(6)
    , department_id  NUMBER(4) 
    ) ;

Example

CREATE UNIQUE INDEX stu_id_pk ON students (student_id) ;
INSERT INTO students VALUES (100,'SMITH','JAMES','SMITH.JAMES@notreal.com','111.111.1245',TO_DATE('17-06-2003','DD-MM-YYYY'),'INS_CHAIRMAN',24000,NULL,NULL);
INSERT INTO students VALUES (101,'JOHNSON','JOHN','JOHNSON.JOHN@notreal.com','111.111.1246',TO_DATE('21-09-2005','DD-MM-YYYY'),'INS_VP',17000,100,90);
INSERT INTO students VALUES (102,'WILLIAMS','ROBERT','WILLIAMS.ROBERT@notreal.com','111.111.1247',TO_DATE('13-01-2001','DD-MM-YYYY'),'INS_VP',17000,100,90);
INSERT INTO students VALUES (103,'BROWN','MICHAEL','BROWN.MICHAEL@notreal.com','111.111.1248',TO_DATE('03-01-2006','DD-MM-YYYY'),'INS_STAFF',9000,102,60);
INSERT INTO students VALUES (104,'JONES','WILLIAM','JONES.WILLIAM@notreal.com','111.111.1249',TO_DATE('21-05-2007','DD-MM-YYYY'),'INS_STAFF',6000,103,60);
INSERT INTO students VALUES (105,'MILLER','DAVID','MILLER.DAVID@notreal.com','111.111.1250',TO_DATE('25-06-2005','DD-MM-YYYY'),'INS_STAFF',4800,103,60);
INSERT INTO students VALUES (106,'DAVIS','RICHARD','DAVIS.RICHARD@notreal.com','111.111.1251',TO_DATE('05-02-2006','DD-MM-YYYY'),'INS_STAFF',4800,103,60);
INSERT INTO students VALUES (107,'GARCIA','CHARLES','GARCIA.CHARLES@notreal.com','111.111.1252',TO_DATE('07-02-2007','DD-MM-YYYY'),'INS_STAFF',4200,103,60);
INSERT INTO students VALUES (108,'RODRIGUEZ','JOSEPH','RODRIGUEZ.JOSEPH@notreal.com','111.111.1253',TO_DATE('17-08-2002','DD-MM-YYYY'),'CL_PHY',12008,101,100);
INSERT INTO students VALUES (109,'WILSON','THOMAS','WILSON.THOMAS@notreal.com','111.111.1254',TO_DATE('16-08-2002','DD-MM-YYYY'),'CL_MATH',9000,108,100);
INSERT INTO students VALUES (110,'MARTINEZ','CHRISTOPHER','MARTINEZ.CHRISTOPHER@notreal.com','111.111.1255',TO_DATE('28-09-2005','DD-MM-YYYY'),'CL_MATH',8200,108,100);
INSERT INTO students VALUES (111,'ANDERSON','DANIEL','ANDERSON.DANIEL@notreal.com','111.111.1256',TO_DATE('30-09-2005','DD-MM-YYYY'),'CL_MATH',7700,108,100);
INSERT INTO students VALUES (112,'TAYLOR','PAUL','TAYLOR.PAUL@notreal.com','111.111.1257',TO_DATE('07-03-2006','DD-MM-YYYY'),'CL_MATH',7800,108,100);
INSERT INTO students VALUES (113,'THOMAS','MARK','THOMAS.MARK@notreal.com','111.111.1258',TO_DATE('07-12-2007','DD-MM-YYYY'),'CL_MATH',6900,108,100);

COMMIT;