Renumbering items in a list with SQL queries? - mysql

Question

The query:
$consulta = "UPDATE `list`
SET `pos` = $pos
WHERE `id_item` IN (SELECT id_item
FROM lists
WHERE pos = '$item'
ORDER BY pos DESC
LIMIT 1)
AND id_usuario = '$us'
AND id_list = '$id_pl'";
The thing is, this query is inside a foreach, and it wants to update the order of the items in a list. Before I had it like this:
$consulta = "UPDATE `list`
SET `pos` = $pos
WHERE `$pos` = '$item'
AND id_usuario = '$us'
AND id_list = '$id_pl'";
But when I update pos 2 -> 1, and then 1 -> 2, the result is two times 2 and no 1...
Is there a solution for this query?

Answer 1

Renumbering the items in a list is tricky. When you renumber the items in the list using multiple separate SQL statements, it is even trickier.
Your inner sub-select statement also is not properly constrained. You need an extra condition such as:
AND id_list = '$id_pl'
There are probably many ways to do this, but the one that may be simplest follows. I'm assuming that:
the unshown foreach loop generates $pos values in the desired sequence (1, 2, ...)
the value of $id_pl is constant for the loop
the foreach loop gives values for $us and $item for each iteration
the combination of $id_pl, $us, and $item uniquely identifies a row in the list table
there aren't more than 100 pos values to worry about
you are able to use an explicit transaction around the statement sequence
The suggested solution has two stages:
Allocate 100 + pos to each row to place it in its new position
Subtract 100 from each pos
This technique avoids any complicated issues about whether rows that have had there position adjusted are reread by the same query.
Inside the loop:
foreach ...
...$pos, $item, $us...
UPDATE list
SET pos = $pos + 100
WHERE id_item = '$item'
AND id_usuario = '$us'
AND id_list = '$id_pl'
AND pos < 100
end foreach
UPDATE list
SET pos = pos - 100
WHERE id__list = '$id_pl';
If you don't know the size of the lists, you could assign negative pos values in the loop and convert to positive after the loop, or any of a number of other equivalent mappings. The key is to update the table so that the new pos numbers in the loop are disjoint from the old numbers, and then adjust the new values after the loop.
Alternative techniques create a temporary table that maps the old numbers to the new and then executes a single UPDATE statement that changes the old pos value to the new for all rows in a single operation. This is probably more efficient, especially if the mapping table can be generated as a query, but that depends on whether the renumbering is algorithmic. The technique shown, albeit somewhat clumsy, can be made to work for arbitrary renumberings.