Recursive query to give a route in postgreSQL - sql

Question

I have to do a recursive function in pSQL to get the following query:
I have a table called tb_route with from_city and to_city
I have another column with the distance in km between different cities.
The table is builded recursively. I have to make a recursive CTE to show the route between two cities (i.e., from 'Santiago de compostela' to 'Saint Jean Pied de Port') showing the total km of the route and the cities where it goes through.
The output has to be something like this:
This is what I've tried:
WITH RECURSIVE cities AS (
SELECT *
FROM textil.tb_route
WHERE to_city_name = 'Santigo de Compostela'
UNION ALL
SELECT e.from_city, e.to_city, e.route, e.km
FROM textil.tb_route e
INNER JOIN cities tb_route ON tb_route.from_city_name = e.from_city
)
SELECT *
FROM cities;
And I had an error like:
ERROR: column e.from_city does not exist
LINE 8: ...JOIN cities tb_route ON tb_route.from_city_name = e.from_cit...
Table:
CREATE TABLE textil.tb_route
(
from_city_name CHARACTER VARYING(120) NOT NULL ,
to_city_name CHARACTER VARYING(120) NOT NULL ,
km_distance_num NUMERIC(5,2) NOT NULL ,
CONSTRAINT pk_route PRIMARY KEY (from_city_name, to_city_name)
);
Data:
INSERT INTO textil.tb_route VALUES
('Saint Jean Pied de Port','Roncesvalles',25.7),
('Somport','Jaca',30.5),
('Roncesvalles','Zubiri',21.5),
('Jaca','Arrés',25),
('Zubiri','Pamplona/Iruña',20.4),
('Arrés','Ruesta',28.7),
('Pamplona/Iruña','Puente la Reina/Gares',24),
('Ruesta','Sangüesa',21.8),
('Puente la Reina/Gares','Estella/Lizarra',22),
('Sangüesa','Monreal',27.25),
('Estella/Lizarra','Torres del Río',29),
('Monreal','Puente la Reina/Gares',31.1),
('Torres del Río','Logroño',20),
('Logroño','Nájera',29.6),
('Nájera','Santo Domingo de la Calzada',21),
('Santo Domingo de la Calzada','Belorado',22.7),
('Belorado','Agés',27.4),
('Agés','Burgos',23),
('Burgos','Hontanas',31.1),
('Hontanas','Boadilla del Camino',28.5),
('Boadilla del Camino','Carrión de los Condes',24.6),
('Carrión de los Condes','Terradillos de los Templarios',26.6),
('Terradillos de los Templarios','El Burgo Ranero',30.6),
('El Burgo Ranero','León',37.1),
('León','San Martín del Camino',25.9),
('San Martín del Camino','Astorga',24.2),
('Astorga','Foncebadón',25.9),
('Foncebadón','Ponferrada',27.3),
('Ponferrada','Villafranca del Bierzo',24.1),
('Villafranca del Bierzo','O Cebreiro',28.4),
('O Cebreiro','Triacastela',21.1),
('Triacastela','Sarria',18.3),
('Sarria','Portomarín',22.4),
('Portomarín','Palas de Rei',25),
('Palas de Rei','Arzúa',28.8),
('Arzúa','Pedrouzo',19.1),
('Pedrouzo','Santiago de Compostela',20),
('Bayona','Ustaritz',14.3),
('Ustaritz','Urdax',21.2),
('Urdax','Elizondo',18.8),
('Elizondo','Berroeta',9.7),
('Berroeta','Olagüe',20.4),
('Olagüe','Pamplona/Iruña',25),
('Irún','Hernani',26.6),
('Hernani','Tolosa',18.9),
('Tolosa','Zerain',33),
('Zerain','Salvatierra/Agurain',28),
('Salvatierra/Agurain','Vitoria/Gasteiz',27.4),
('Vitoria/Gasteiz','La Puebla de Arganzón',18.5),
('La Puebla de Arganzón','Haro',31),
('Haro','Santo Domingo de la Calzada',20),
('Bayona','Irún',33.8),
('Tolosa','Zegama',37.9),
('Zegama','Salvatierra/Agurain',20.1),
('La Puebla de Arganzón','Miranda del Ebro',22.3),
('Miranda del Ebro','Pancorbo',16.7),
('Pancorbo','Briviesca',23.4),
('Briviesca','Monasterio de Rodilla',19.8),
('Monasterio de Rodilla','Burgos',28.5);
```

Answer 1

Here I leave the solution I've get finally:
with recursive caminos(from_city_name, to_city_name, path, total_distance, terminar, ultima_ciudad) as (
-- Consulta base
select to_city_name
,'Saint Jean Pied de Port' -- Cambiar Destino
,concat(to_city_name, concat(' -> ', from_city_name))
,cast(km_distance_num as numeric(8,2))
,0 --No terminar
,from_city_name
from textil.tb_route
where to_city_name = 'Santiago de Compostela' -- Cambiar Origen
union all
-- Consulta recursiva
select caminos.from_city_name
,caminos.to_city_name
,concat(caminos.path, concat( ' -> ', tr.from_city_name))
,cast(caminos.total_distance + tr.km_distance_num as numeric(8,2))
,case when tr.from_city_name = caminos.to_city_name then 1 else 0 end
,tr.from_city_name
from caminos inner join textil.tb_route tr on tr.to_city_name = caminos.ultima_ciudad and caminos.terminar != 1
)
select from_city_name, to_city_name, path, total_distance
from caminos
where 1 = 1
and from_city_name = 'Santiago de Compostela' --Cambiar Origen
and ultima_ciudad = 'Saint Jean Pied de Port' -- Cambiar Destino
;

Answer 2

I understand your question as a graph-walking problem. As described in your questions, edges are directed (meaning that you can travel from from_city_name to to_city_name, but not the other way around).
Here is an approach using a recursive CTE. The idea is to start from a given city, and then follow all possible routes, while keeping track of the overall travel path in an arry. The recursion stops either when a circle is detected, or when the target city is reached. Then, the outer query filters on the successful paths (there may be none, one or several).
with recursive cte as (
select
from_city_name,
to_city_name,
km_distance_num,
array[from_city_name::text, to_city_name::text] path
from tb_route
where from_city_name = 'Saint Jean Pied de Port'
union all
select
r.from_city_name,
r.to_city_name,
c.km_distance_num + r.km_distance_num,
c.path || r.to_city_name::text
from tb_route r
inner join cte c on c.to_city_name = r.from_city_name
where
not r.to_city_name = any(c.path)
and c.from_city_name <> 'Santiago de Compostela'
)
select
path[1] from_city_name,
to_city_name,
km_distance_num,
array_to_string(path, ' > ') path
from cte
where to_city_name = 'Santiago de Compostela';
Demo on DB Fiddle:
from_city_name | to_city_name | km_distance_num | path
:---------------------- | :--------------------- | --------------: | :------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Saint Jean Pied de Port | Santiago de Compostela | 775.3 | Saint Jean Pied de Port > Roncesvalles > Zubiri > Pamplona/Iruña > Puente la Reina/Gares > Estella/Lizarra > Torres del Río > Logroño > Nájera > Santo Domingo de la Calzada > Belorado > Agés > Burgos > Hontanas > Boadilla del Camino > Carrión de los Condes > Terradillos de los Templarios > El Burgo Ranero > León > San Martín del Camino > Astorga > Foncebadón > Ponferrada > Villafranca del Bierzo > O Cebreiro > Triacastela > Sarria > Portomarín > Palas de Rei > Arzúa > Pedrouzo > Santiago de Compostela