Reordering of column data in mysql

Question

I have a table like so:

categoryID      categoryName
----------------------------
     1            A
     2            B
     3            C

Answer 1

If the number of changes is rather small you can generate a clumsy but rather efficient UPDATE statement if the you know the ids of the involved items:

UPDATE categories
JOIN (
    SELECT 2 as categoryID, 3 as new_order
    UNION ALL
    SELECT 3 as categoryID, 4 as new_order
    UNION ALL
    SELECT 4 as categoryID, 2 as new_order) orders
USING (categoryId)
SET `order` = new_order;

or (which I like less):

UPDATE categories
SET `order` = ELT (FIND_IN_SET (categoryID, '2,3,4'),
                   3, 4, 2)
WHERE categoryID in (2,3,4);

UPD:

Assuming that you know the current id of the category (or its name), its old position, and its new position you can use the following query for moving a category down the list (for moving up you will have to change the between condition and new_rank computation to rank+1):

SET @id:=2, @cur_rank:=2, @new_rank:=4;

UPDATE t1
JOIN (
  SELECT categoryID, (rank - 1) as new_rank
  FROM t1
  WHERE rank between @cur_rank + 1 AND @new_rank
  UNION ALL
  SELECT @id as categoryID, @new_rank as new_rank
) as r
USING (categoryID)
SET rank = new_rank;