SQLite Forum

cycle detection in CHECK constraint with recursive CTE
Hello, here's is the trigger code I ended up using:

CREATE TRIGGER cycle_check
BEFORE INSERT ON tag_parents
    SELECT RAISE(ABORT, 'Cycle detected') WHERE EXISTS (
      WITH RECURSIVE w(parent, last_visited, already_visited, cycle) AS (
            SELECT DISTINCT to_tag_id AS parent, from_tag_id AS last_visited, to_tag_id AS already_visited, 0 AS cycle FROM tag_parents

            UNION ALL

            SELECT t.to_tag_id AS parent, t.from_tag_id AS last_visited, already_visited || ', ' || t.to_tag_id, already_visited LIKE '%'||t.to_tag_id||"%" FROM tag_parents AS t JOIN w ON w.last_visited = t.to_tag_id
            WHERE NOT cycle
      SELECT already_visited, cycle FROM w WHERE last_visited = NEW.to_tag_id AND already_visited LIKE '%'||NEW.from_tag_id||'%'

I hope it proves useful (and correct) for anyone with a similar problem. I made a small writeup [here](https://sic.nessuent.xyz/s/18/detecting-cycles-in-tag-parent-child-relationship-in-sqlite3/) with a tiny bit more detail if anyone is interested.