Recursion in PostgreSQL - sql

Question

Let's assume we have a table borders(country1,country2) that contains two countries that border each other, eg. (Sweden, Norway), etc. I would like to find all the countries that can be reached from a given country, say Sweden, by using border crossing only.
Here's the first part of my solution:
WITH RECURSIVE border(countryin) AS (
select distinct country
from (select country2::character varying(4) as country
from borders where country1 = 'S'
union
select country1::character varying(4) as country
from borders where country2 = 'S' ) a
UNION
select distinct sp.country::varchar(4)
from (select country1::varchar(4) as country, country2 as n
from borders) sp
join (select country2::varchar(4) as country, country1 as n, countryin as temp
from borders, border) st
on sp.country = st.n
and sp.country in st.temp
where true
)
SELECT distinct countryin, name
FROM border, country
where countryin = code ;
The only thing that I cannot get to work is how to set a constraint so that a specific country exists in the result border table. I tried using and sp.country in st.temp, and several other ways, but I cannot get it to work.
Could some one give me a hint of how this can be solved?
Current Results:
Right now, I get an error stating "
ERROR: syntax error at or near "st"
LINE 4: ...s, border) st on sp.country = st.n and sp.country in st.temp
"
Desired Results
List all counties that can be reached recursively using borders starting from 'S'. So, if we have (S,N), (N,R), (R,C), (D,A), we would get: (N,R,C)

Answer 1

I believe there is room for improvement, but seem like do the job.
base case, you get the "other" country where 'S' appear in any side
recursive case get new country with border with any country already in travel path, but avoid the one with 'S' so doesnt return to origin. Also include a variable to track recursive depth so doesnt keep looping for ever. (dont remember how many country are now).
After finish I add filter DISTINCT to remove duplicate.
Maybe I could include a filter on the recursive case To avoid travel back to same countries. Not sure which one is more efficient.
AND ( b.country1 NOT IN (SELECT country FROM Travel)
AND b.country2 NOT IN (SELECT country FROM Travel)
)
SQL Fiddle DEMO
WITH RECURSIVE travel(r_level, country) AS (
select distinct 1 as r_level,
CASE WHEN country1 = 'S' THEN country2
ELSE country1
END as country
from borders
where country1 = 'S'
or country2 = 'S'
UNION
select distinct t.r_level + 1 as r_level,
CASE WHEN b.country1 = t.country THEN b.country2
ELSE b.country1
END as country
from borders b
join travel t
ON (b.country1 = t.country OR b.country2 = t.country)
AND (b.country1 <> 'S' AND b.country2 <> 'S')
WHERE t.r_level < 300
)
SELECT DISTINCT country
FROM travel
OUTPUT
| country |
|---------|
| N |
| R |
| C |
Please feel free to provide a more complete sqlFiddle with more country to improve the testing.