How do I trace former ids using a recursive query? - sql

Question

I have a table of provider information (providers) that contains the columns reporting_unit and predesessor. Predesessor is either
null or contains the reporting_unit that that row used to represent. I need
to find what the current reporting_unit for any provider is. By that I mean for any reporting_unit with a predesessor, that reporting_unit is the current_reporting_unit for the predesessor.
I am trying
to use a recursive CTE to accomplish this because some of the time
there are multiple links.
The table looks like this:
CREATE TABLE providers (
reporting_unit TEXT,
predesessor TEXT
);
INSERT INTO providers
VALUES
(NULL, NULL),
('ARE88', NULL),
('99BX7', '99BX6'),
('99BX6', '99BX5'),
('99BX5', NULL)
;
The results I would like to get from that are:
reporting_unit | current_reporting_unit
---------------------------------------
'99BX5' | '99BX7'
'99BX6' | '99BX7'
My current query is :
WITH RECURSIVE current_ru AS (
SELECT reporting_unit, predesessor
FROM providers
WHERE predesessor IS NULL
UNION ALL
SELECT P.reporting_unit, P.predesessor
FROM providers P
JOIN current_ru CR
ON P.reporting_unit = CR.predesessor
)
SELECT *
FROM current_ru
;
But that isn't giving me the results I'm looking for. I have tried a number of variations on this query but they all seem to end up in an infinite loop. How

Answer 1

You should find relations in the reverse order. Add depth column to find the deepest link:
with recursive current_ru (reporting_unit, predesessor, depth) as (
select reporting_unit, predesessor, 1
from providers
where predesessor is not null
union
select r.reporting_unit, p.predesessor, depth+ 1
from providers p
join current_ru r
on p.reporting_unit = r.predesessor
)
select *
from current_ru;
reporting_unit | predesessor | depth
----------------+-------------+-------
99BX7 | 99BX6 | 1
99BX6 | 99BX5 | 1
99BX6 | | 2
99BX7 | 99BX5 | 2
99BX7 | | 3
(5 rows)
Now switch the two columns, change their names, eliminate null rows and select the deepest links:
with recursive current_ru (reporting_unit, predesessor, depth) as (
select reporting_unit, predesessor, 1
from providers
where predesessor is not null
union
select r.reporting_unit, p.predesessor, depth+ 1
from providers p
join current_ru r
on p.reporting_unit = r.predesessor
)
select distinct on(predesessor)
predesessor reporting_unit,
reporting_unit current_reporting_unit
from current_ru
where predesessor is not null
order by predesessor, depth desc;
reporting_unit | current_reporting_unit
----------------+------------------------
99BX5 | 99BX7
99BX6 | 99BX7
(2 rows)