Connect by in Oracle SQL

Question

Suppose that we have following tables

create table Employee(
  2    EMPNO         NUMBER(3),
  3    ENAME         VARCHAR2(15 BYTE),
  4    HIREDATE      DATE,
  5    ORIG_SALARY   NUMBER(6),
  6    CURR_SALARY   NUMBER(6),
  7    REGION        VARCHAR2(1 BYTE),
  8    MANAGER_ID    NUMBER(3)
  9  )

and

create table job (
  2    EMPNO         NUMBER(3),
  3    jobtitle      VARCHAR2(20 BYTE)
  4  )
  5  /

I am interested in what the below query does

SELECT empno, manager_id, ename
  2  FROM employee
  3  START WITH empno = 1
  4  CONNECT BY PRIOR empno = manager_id;

As I understood this code selects empno,manager_id,ename from employee, it starts selection from this row where empno=1, but could not understand what this line does:

CONNECT BY PRIOR empno = manager_id;

Is this the same as:

where empno=manager_id?

Amide

Docs – Downwash

Answer 1

CONNECT BY PRIOR empno = manager_id;

This will produce the recursion. All records that are part of the next lower hierarchical level will return. This will return a hierarchy from top to bottom for all managers and their respective under working subordinates.

30 (manager_id)
   12 
   5 (manager_id)
      1
      7
20 (manager_id)
   15
   10

Fournier

so how can i guess if i need code like this to select something? – Amide

Simply when ever you want to get something which has parent-child relation in hierarchical manner. – Fournier

Answer 2

The query is recursive, it start from employee #1 (CEO probably) and then recursively prints all his subordinates and then all their subordinates and so on and so forth (until all the employees are printed).

A good explanation about the "START WITH" and "CONNECT BY" can be found here

Hannigan