MySQL trigger to update a column by reordering its values - mysql

Question

Sorry if the title is miss-leading, I couldn't come up with a better one that is related to my issue.
I've been trying to solve this for a while now, and I couldn't find the solution.
I have a table categories:
+----+--------+----------+
| ID | Name | Position |
+----+--------+----------+
| 1 | Dogs | 4 |
| 2 | Cats | 3 |
| 3 | Birds | 10 |
| 4 | Others | 2 |
+----+--------+----------+
I need to keep the Position column in order, in a way not to miss an values as well, so the final table should look like:
+----+--------+----------+
| ID | Name | Position |
+----+--------+----------+
| 1 | Dogs | 3 |
| 2 | Cats | 2 |
| 3 | Birds | 4 |
| 4 | Others | 1 |
+----+--------+----------+
What I tried doing, is creating a trigger on UPDATE and on INSERT that would try to prevent this. The trigger I created ( same one before INSERT) :
DELIMITER //
CREATE TRIGGER sortPostions BEFORE UPDATE ON categories
FOR EACH ROW BEGIN
SET #max_pos = 0;
SET #min_pos = 0;
SET #max_ID = 0;
SET #min_ID = 0;
SELECT position, id INTO #max_pos,#max_ID FROM categories WHERE position = ( SELECT MAX(position) FROM categories);
SELECT position, id INTO #min_pos,#min_ID FROM categories WHERE position = ( SELECT MIN(position) FROM categories);
IF NEW.position >= #max_pos AND NEW.id != #max_ID THEN
SET NEW.position = #max_pos + 1;
END IF;
IF NEW.position <= #min_pos AND NEW.id != #min_ID THEN
SET NEW.position = #min_pos - 1;
END IF;
IF NEW.position < 0 THEN
SET NEW.position = 0;
END IF;
END//
DELIMITER ;
But unfortunately it's not working as intended. It's not fixing missing values and I think this is not a perfect solution.
I went ahead and created a procedure:
BEGIN
SET #n = 0;
UPDATE categories
SET position = #n:=#n+1
ORDER BY position ASC;
END
But I wasn't able to call this procedure from a trigger, as it seems that MySQL doesn't allow that. I get the following error:
#1442 - Can't update table 'categories' in stored function/trigger because it is already used by statement which invoked this stored function/trigger.
mysql -V output:
mysql Ver 14.14 Distrib 5.5.57, for debian-linux-gnu (x86_64) using readline 6.3
What's the perfect solution to solve this problem ?
Thanks a lot!

Answer 1

You can't do this in a trigger. MySQL does not allow you to do an INSERT/UPDATE/DELETE in a trigger (or in a procedure called by the trigger) against the same table for which the trigger was spawned.
The reason is that it can result in infinite loops (your update spawns the trigger, which updates the table, which spawns the trigger again, which updates the table again...). Also because it can create lock conflicts if more than one of these requests happens concurrently.
You should do this in application code if you must renumber the position of the rows. Do it with a separate statement after your initial query has completed.
Another option is don't worry about making the positions consecutive. Just make sure they are in the right order. Then when you query the table, generate row numbers on demand.
SELECT (#n:=#n+1) AS row_num, c.*
FROM (SELECT #n:=0 AS n) AS _init
CROSS JOIN categories AS c
ORDER BY c.position ASC;
+---------+----+--------+----------+
| row_num | id | name | position |
+---------+----+--------+----------+
| 1 | 4 | Others | 2 |
| 2 | 2 | Cats | 3 |
| 3 | 1 | Dogs | 4 |
| 4 | 3 | Birds | 10 |
+---------+----+--------+----------+
In MySQL 8.0, you'll be able to do this with more standard syntax using ROW_NUMBER().
SELECT ROW_NUMBER() OVER w AS row_num, c.*
FROM categories AS c
WINDOW w AS (ORDER BY c.position ASC);
Gives the same output as the query using the user-variable.