Find sql connected component between many to many entities - sql

Question

I have two basic entities: financial plan and purchase request. Theese two entities are in many-to-many relationship:
CREATE TABLE FinancialPlan
(
ID int NOT NULL,
PRIMARY KEY (ID)
);
CREATE TABLE PurchaseRequest
(
ID int NOT NULL,
PRIMARY KEY (ID)
);
CREATE TABLE FP_PR
(
FP_ID FOREIGN KEY REFERENCES FinancialPlan(ID),
PR_ID FOREIGN KEY REFERENCES PurchaseRequest(ID)
);
Problem: find all requests, related to specified plan, and all plans, related to requests, related to specified plan, ...
Model could be represented as a graph, where each node represents a plan, or a request, and each edge represents a relationship, then the problem could be rephrased as find connected component, which specified node belongs to.
Example:
Plan Request FP_PR
ID | ID | FP_ID|PR_ID|
----| ----| -----|-----|
1 | 1 | 1 |1 |
2 | 2 | 2 |1 |
3 | 3 | 2 |2 |
4 | 3 |2 |
5 | 4 |2 |
5 |3 |
Find connected component of finplan ID=1
Desired output:
FP_ID | PR_ID|
------+------+
1 | 1 |
2 | 1 |
2 | 2 |
3 | 2 |
4 | 2 |
I am currently doing it recursively on app side, which may generate to many requests and hang the DB server, could this be done with some recursive DB approach?
Visualization:
Starting entity is marked by arrow.
Desired output is circled.

Answer 1

SQL Server solution
I guess the main problem is you need to compare by PR_ID then FP_ID. So in recursive part there must be a CASE statement. On 1 run we take data by FP_ID on second by PR_ID and etc with the help of modulo.
DECLARE #fp int = 1
;WITH cte AS (
SELECT f.FP_ID,
f.PR_ID,
1 as lev
FROM #FP_PR f
WHERE f.FP_id = #fp
UNION ALL
SELECT f.FP_ID,
f.PR_ID,
lev+1
FROM cte c
CROSS JOIN #FP_PR f -- You can use INNER JOIN instead
WHERE CASE (lev+1)%2 WHEN 0 THEN f.PR_ID WHEN 1 THEN f.FP_ID END = CASE (lev+1)%2 WHEN 0 THEN c.PR_ID WHEN 1 THEN c.FP_ID END
AND NOT (f.PR_ID = c.PR_ID AND f.FP_ID = c.FP_ID)
)
SELECT *
FROM cte
Output:
FP_ID PR_ID lev
1 1 1
2 1 2
2 2 3
3 2 4
4 2 4