MySql Update Row and it's "Descendants" - mysql

Question

I'm having trouble figuring out how to update the descendants/children of a row.
Example table, named test
+----+--------+------+
| Id | Parent | Val |
+----+--------+------+
| 1 | 0 | |
| 2 | 1 | |
| 3 | 1 | |
| 4 | 1 | |
| 5 | 0 | |
| 6 | 5 | |
| 7 | 6 | |
| 8 | 6 | |
| 9 | 0 | |
+----+--------+------+
What I'd like to have done is, when Val is set to something, update every row that is related to that row. For example, If I ran
UPDATE test SET Val=1 WHERE Id=5;
I want Val in the rows where the Id is 6, 7, and 8, to also be 1.
The best thing I could come up with was
UPDATE test t1
JOIN test t2 ON t1.Id = t2.Parent
JOIN test t3 ON t2.Id = t3.Parent
SET t1.Val=1, t2.Val=1, t3.Val=1 WHERE t1.Id=5;
+----+--------+------+
| Id | Parent | Val |
+----+--------+------+
| 1 | 0 | |
| 2 | 1 | |
| 3 | 1 | |
| 4 | 1 | |
| 5 | 0 | 1 |
| 6 | 5 | 1 |
| 7 | 6 | 1 |
| 8 | 6 | 1 |
| 9 | 0 | |
+----+--------+------+
This gives me what I want, but I'm afraid it's poor practice and it doesn't account for a variable depth. What can I do here? I thought a trigger may have been the answer, but that didn't seem to be possible. I got the error "Can't update table in stored function/trigger because it is already used by statement which invoked this stored function/trigger"

Answer 1

It's pretty awkward to handle this kind of case with MySQL, because MySQL doesn't support recursive queries.
I solve this problem by storing the hierarchy as a transitive closure, instead of the "parent_id" style you're using. See my answer to What is the most efficient/elegant way to parse a flat table into a tree?
Then you can update all descendants of a given node in the heirarchy:
UPDATE test JOIN testclosure AS c ON test.id = c.descendant
SET test.val = 1
WHERE c.ancestor = 5;