Simple recursive query in Oracle

Question

I\'m currently having some trouble understanding and writing recursive queries. I understand that recursive queries are used to search through hierarchies of information, bu

Answer 1

Are you familiar with the SCOTT.EMP table? It's in the "standard" SCOTT schema (which, unfortunately, is no longer pre-packaged with every copy of Oracle database, since version 12.1 or so). Check your database: you may find it there. Or ask your DBA about it.

Anyway: the table shows the 14 employees of a small business, and it includes the employee's ID as well as his or her manager's employee ID. So, suppose you start with a given employee and you want to find his or her highest-level boss. (Similar to your test problem.) In this particular hierarchy, the highest-level "ancestor" is unique, but that is irrelevant; the recursive query would work the same way if each department had a "head of department" and there was no CEO above the heads of department.

In this arrangement, it's easy to identify the "boss of all bosses" - he does not have a boss. In his row, the manager ID is null. This is a very common arrangement for the "root" (or "roots") of tree-like hierarchies.

Here is how you would find the boss, starting with a specific employee id, and using a recursive query - which is what I understand is what you are looking to practice on. (That is: if I understand correctly, you are not interested in solving the problem "by any means"; rather, you want to see how recursive queries work, in a small example so you can understand EVERYTHING that goes on.)

with
  r ( empno, mgr ) as (
    select  empno, mgr      -- ANCHOR leg of recursive query
      from  scott.emp
      where empno = 7499
    union all
    select  e.empno, e.mgr  -- RECURSIVE leg of recursive query
      from  scott.emp e inner join r on e.empno = r.mgr
  )
select empno
from   r
where  mgr is null
;

I will not try to guess where you may have difficulty understanding this example. Instead, I will wait for you to ask.