Subject: Re: Does PostgreSQL have a pseudo-column like "LEVEL" in Oracle
To: pgsql-sql@lists.postgresql.org
References: 
 <CAMV54g0xEw_xoj0p2hmDio6fN5bPyybSp0oXOntrpzAN_FUOhg@mail.gmail.com>
From: Thomas Kellerer <shammat@gmx.net>
Message-ID: <205bcd0b-d74b-b686-ec31-76a04c279a6d@gmx.net>
Date: Sun, 17 Oct 2021 12:53:56 +0200
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; de; rv:1.8.1.21)
 Gecko/20090302 Thunderbird/2.0.0.21 Mnenhy/0.7.5.666
MIME-Version: 1.0
In-Reply-To: 
 <CAMV54g0xEw_xoj0p2hmDio6fN5bPyybSp0oXOntrpzAN_FUOhg@mail.gmail.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: de-DE
Content-Transfer-Encoding: quoted-printable
Archived-At: 
 <https://www.postgresql.org/message-id/205bcd0b-d74b-b686-ec31-76a04c279a6d%40gmx.net>
Precedence: bulk


Jian He schrieb am 17.10.2021 um 10:36:
> https://stackoverflow.com/questions/22626394/does-postgresql-have-a-pseu=
do-column-like-level-in-oracle
> Wandering around, playing around, then problems come.  I tried to
> crack the *level *concept . So I followed through with the most voted
> answer from the above link.  The following is my code sample data.

>
> begin;
> create temptable tempemp  (employee_idinteger primary key, last_name tex=
t,manager_idinteger);
> insert into tempempvalues(1,'eliane',1);
> insert into tempempvalues(2,'sponge',1);
> insert into tempempvalues(3,'george',1);
> insert into tempempvalues(4,'kramer',2);
> insert into tempempvalues(5,'megan',2);
> insert into tempempvalues(6,'donald',3);
> commit;
>
> WITH RECURSIVE cteAS (
>     SELECT employee_id, last_name, manager_id,1 AS level
> FROM tempemp
>
>     UNION ALL
> SELECT e.employee_id, e.last_name, e.manager_id, c.level +1
> FROM cte c
>     JOIN tempemp eON e.manager_id =3D c.employee_id
>     )
> SELECT *
> FROM cte;
>


This row:

> insert into tempempvalues(1,'eliane',1);

creates an endless loop because it points to itself.
If there is no manager assigned you should use NULL instead

Additionally your recursive CTE does not have a condition for the starting=
 element

If you want to stick with the circular reference of an employee to itself,=
 you need
to exlcude the starting element

       WITH RECURSIVE cte AS (
          SELECT employee_id, last_name, manager_id, 1 AS level
          FROM   tempemp
          where employee_id =3D 1

          UNION  ALL

          SELECT e.employee_id, e.last_name, e.manager_id, c.level + 1
          FROM   tempemp e
           JOIN   cte c ON e.manager_id =3D c.employee_id
          where e.employee_id <> 1
       )
       SELECT *
       FROM   cte;


But it would be better to use:

     insert into tempempvalues(1,'eliane', null);

Then you don't need to exclude the root element in the recursive part:

       WITH RECURSIVE cte AS (
          SELECT employee_id, last_name, manager_id, 1 AS level
          FROM   tempemp
          where manager_id is null

          UNION  ALL

          SELECT e.employee_id, e.last_name, e.manager_id, c.level + 1
          FROM   tempemp e
           JOIN   cte c ON e.manager_id =3D c.employee_id
       )
       SELECT *
       FROM   cte;