SQL Server Recursive CTE for finding all the dependencies - sql

Question

I have a table with the following data.
Road City
R1 C1
R2 C2
R3 C1
R3 C3
R4 C3
R4 C5
R5 C5
If R1 is the input I need to get R1, R3, R4 and R5 as the output. This is because R1 belongs to C1 and C1 has R3 and R3 also belongs to C3 which has R4 and similarly R5.
I was trying to make use of CTE recursion but not able to get it to work. I tried stored procedure recursive call but it goes only 30 levels deep.
with tmp1 as (
select ROAD, CITY, 1 as Level from table R1 WHERE ROAD = 1712
UNION ALL
select R2.ROAD, R2.CITY,Level + 1 as Level
from tmp1 INNER JOIN table R2 ON tmp1.CITY = R2.CITY and tmp1.ROAD <> R2.ROAD
)
select * from tmp1
OPTION (maxrecursion 0)
Any thoughts greatly appreciated!

Answer 1

A recursive CTE will not work without some way of breaking cycles. Other database vendors have specific features for disallowing a row to be added twice. Unless something was added in the latest releases, Microsoft SQL Server does not.
The following does not work, because the recursive clause is referring to the CTE twice. (Or it contains a subquery)
WITH recur AS (SELECT Road, City
FROM #Map
WHERE Road = #StartingRoad
--
UNION ALL
--
SELECT next.Road, next.City
FROM #Map next
INNER JOIN recur
ON (recur.City = next.City AND recur.Road <> next.Road)
OR (recur.City <> next.City AND recur.Road = next.Road)
WHERE NOT EXISTS (SELECT NULL
FROM recur test
WHERE test.Road = next.Road AND test.City = next.City))
SELECT *
FROM recur;
Msg 253, Level 16, State 1, Line 36
Recursive member of a common table expression 'recur' has multiple recursive references.
It is possible with a straight forward loop, which you could stick in a stored procedure:
DECLARE #Map TABLE (Road VARCHAR(2), City VARCHAR(2));
INSERT INTO #Map (Road, City)
VALUES ('R1', 'C1')
, ('R2', 'C2')
, ('R3', 'C1')
, ('R3', 'C3')
, ('R4', 'C3')
, ('R4', 'C5')
, ('R5', 'C5');
DECLARE #StartingRoad VARCHAR(2) = 'R1';
DECLARE #Results TABLE (Road VARCHAR(2), City VARCHAR(2));
INSERT INTO #Results (Road, City)
SELECT Road, City
FROM #Map
WHERE Road = #StartingRoad
WHILE (1=1) BEGIN
INSERT INTO #Results (Road, City)
SELECT next.Road, next.City
FROM #Map next
INNER JOIN #Results r
ON (r.City = next.City AND r.Road <> next.Road)
OR (r.City <> next.City AND r.Road = next.Road)
WHERE NOT EXISTS (SELECT NULL
FROM #Results test
WHERE test.Road = next.Road AND test.City = next.City);
IF ##ROWCOUNT = 0
BREAK;
END;
SELECT DISTINCT Road
FROM #Results

Answer 2

This might get you partially there. I am doing a partial Cartesian join to generate the city/road combinations, and artificially restricting the recursion to 20 levels. I can't help but think there is a better way.
WITH cte(road,
city,
connected_city,
connected_road)
AS (
SELECT a.road,
a.city,
b.city AS connected_city,
b.Road connected_road
FROM deleteme a
INNER JOIN deleteme b ON a.City = b.City
WHERE a.road <> b.road)
SELECT DISTINCT
road
FROM cte;
road
R1
R3
R4
R5