Sequentially update mysql table - mysql

Question

Given the following table, how can I sequentially reorder position from 1 to N using a single query after one or more rows have been deleted and still preserve the order of position?
+---------+----------+-----+
| id (pk) | position | fk |
+---------+----------+-----+
| 4 | 1 | 123 |
| 2 | 2 | 123 |
| 18 | 3 | 123 |
| 5 | 4 | 123 |
| 3 | 5 | 123 |
+---------+----------+-----+
For instance, if position=1 (id=4) was deleted, the desired final records are:
+---------+----------+-----+
| id (pk) | position | fk |
+---------+----------+-----+
| 2 | 1 | 123 |
| 18 | 2 | 123 |
| 5 | 3 | 123 |
| 3 | 4 | 123 |
+---------+----------+-----+
and if position=3 (id=18) was deleted, the desired final records are:
+---------+----------+-----+
| id (pk) | position | fk |
+---------+----------+-----+
| 4 | 1 | 123 |
| 2 | 2 | 123 |
| 5 | 3 | 123 |
| 3 | 4 | 123 |
+---------+----------+-----+
I can do something like the following if only row was deleted but not for multiple rows.
DELETE FROM mytable WHERE fk=123 AND position = 4;
UPDATE mytable SET position=position-1 WHERE fk=123 AND position > 4;

Answer 1

User-defined variables to the rescue if you're not already using MySQL 8, which provides window functions like ROW_NUMBER():
UPDATE t
JOIN (
SELECT
t.*
, #n := #n + 1 as n
FROM t
, (SELECT #n := 0) var_init
ORDER BY position
) sq ON t.id = sq.id
SET t.position = sq.n;
see it working live in an sqlfiddle
BONUS:
It gets slightly more complicated, when you have multiple groups.
For example, for sample data like this
| id | position | fk |
|-----|----------|-----|
| 4 | 1 | 123 |
| 2 | 2 | 123 |
| 5 | 4 | 123 |
| 3 | 5 | 123 |
| 40 | 1 | 234 |
| 20 | 2 | 234 |
| 180 | 3 | 234 |
| 30 | 5 | 234 |
the query would be
UPDATE t
JOIN (
SELECT
t.*
, #n := if(#prev_fk != fk, 1, #n + 1) as n
, #prev_fk := fk
FROM t
, (SELECT #n := 0, #prev_fk := NULL) var_init
ORDER BY fk, position
) sq ON t.id = sq.id
SET t.position = sq.n;
Here you just save the current fk in another variable. When the next row is processed, the variable still holds the value of the "previous row". Then you reset the #n variable, when the value changes.
see it working live in an sqlfiddle
UPDATE:
In MySQL 8 you can use the window function row_number() like this:
update t join (
select t.*, row_number() over (partition by fk order by position) as new_pos
from t
) sq using (id) set t.position = sq.new_pos;

Answer 2

You can use update and the ROW_NUMBER() function. If you order by position you it should be ok.
UPDATE [1]
SET POSITION = [2].RN
FROM t [1]
JOIN (
SELECT
t.ID
, ROW_NUMBER() OVER (ORDER BY POSITION DESC) AS RN
FROM t
) [2]
ON [1].id = [2].id
As people have mentioned this is not applicable for MySql. Sorry for the missinformation as i didnt saw the tag.