Renumber keys in compound unique constraint - mysql

Question

I have a table that looks something like the following:
| id | sub_id | fk_id |
|----|--------|-------|
| 1 | 1 | 1 |
| 2 | 2 | 1 |
| 3 | 3 | 1 |
| 4 | 4 | 1 |
| 5 | 5 | 1 |
| 6 | 1 | 2 |
| 7 | 2 | 2 |
| 8 | 3 | 2 |
| 9 | 4 | 2 |
| 10 | 5 | 2 |
Within this table id is the primary key, and sub_id and fk_id make up a compound unique key, where fk_id is the primary key in another table.
I've found myself in the situation where I need to be able to remove rows within the table, but then renumber sub_id so that there aren't any gaps, e.g. remove (1, 1, 1) and all rows where fk_id=1 have their respective sub_id renumbered as 1, 2, 3, 4, etc.
I also need to be able to remove one or more rows at a time, then trigger the re-numbering (as I assume it's inefficient to try and renumber them multiple times when once will suffice). However, there's a maximum of 60 rows for each value of fk_id but there can be thousands of different values of fk_id.
How should I go about the re-numbering? I'm think some sort of INSERT ... SELECT query, but I can't get my head around how it should work.

Answer 1

You can renumber the rows for a given fk_id using this query:
select t_renum.*, count(t_lower.id) as new_sub_id
from mytable t_renum
join mytable t_lower
on t_lower.fk_id = t_renum.fk_id
and t_lower.id <= t_renum.id
where t_renum.fk_id = #renumber_fk_id
group by t_renum.id
The result can be joined with the original table for update like this:
update mytable t
join (
select t_renum.*, count(t_lower.id) as new_sub_id
from mytable t_renum
join mytable t_lower
on t_lower.fk_id = t_renum.fk_id
and t_lower.id <= t_renum.id
where t_renum.fk_id = #renumber_fk_id
group by t_renum.id
) t_renum using (id)
set t.sub_id = t_renum.new_sub_id
sqlfiddle

Answer 2

After digging around more, I discovered another answer that was remarkably simple and avoided the need for a new table which is recommended in many similar questions. I converted it to a stored procedure which suits my needs better:
DELIMITER //
CREATE PROCEDURE reindex (IN fk_key INT UNSIGNED)
BEGIN
SET #num := 0;
UPDATE example
SET sub_id = (#num := #num + 1)
WHERE fk_id = fk_key
ORDER BY id;
END //
DELIMITER ;