Updating column so that it contains the row position - php

Question

This is the content table:
ContentID | CategoryID | Position | Other1 | Other2
===================================================
1 | 1 | NULL | abcd | efgh
2 | 1 | NULL | abcd | efgh
3 | 1 | NULL | abcd | efgh
4 | 2 | NULL | abcd | efgh
5 | 2 | NULL | abcd | efgh
6 | 2 | NULL | abcd | efgh
These are the queries I'll be running:
SELECT ContentID FROM content WHERE CategoryID = 1 ORDER BY Position
SELECT ContentID FROM content WHERE CategoryID = 2 ORDER BY Position
Now I want to implement move up, move down, move to top and move to bottom function for content. All I need to do is to populate the Position column with numbers:
ContentID | CategoryID | Position
=================================
1 | 1 | 1
2 | 1 | 2
3 | 1 | 3
4 | 2 | 1
5 | 2 | 2
6 | 2 | 3
Is it possible to achieve this via single query in MySQL? Something like:
UPDATE content
SET Position = <ROW_NUMBER>
WHERE CategoryID = 1
ORDER BY Position
UPDATE content
SET Position = <ROW_NUMBER>
WHERE CategoryID = 2
ORDER BY Position

Answer 1

This should work
update
content,
(
select
#row_number:=ifnull(#row_number, 0)+1 as new_position,
ContentID
from content
where CategoryID=1
order by position
) as table_position
set position=table_position.new_position
where table_position.ContentID=content.ContentID;
But I would prefer to apply this first, to unset user defined variable
set #row_number:=0;
Added by Mchl:
You can do that in one statement like this
update
content,
(
select
#row_number:=ifnull(#row_number, 0)+1 as new_position,
ContentID
from content
where CategoryID=1
order by position
) as table_position,
(
select #row_number:=0
) as rowNumberInit
set position=table_position.new_position
where table_position.ContentID=content.ContentID;

Answer 2

Here is the solution that worked for me (hope it helps someone):
-- The following query re-populates the "Position" column with sequential numbers so that:
-- a) sequence is reset to 1 for each "group"
-- b) sequence is based on row number relative to each group depending on how ORDER BY is specified
-- c) sequence does not disturb the original order but
-- c.a) fixes NULLs so that they are moved to top
-- c.b) fixes duplicate position values depending on how ORDER BY is specified
-- ContentID is the primary key
-- CategoryID is a foreign key
-- Position column contains relative position of a record
SET #current_group = NULL;
SET #current_count = NULL;
UPDATE
content
SET Position = CASE
WHEN #current_group = CategoryID THEN #current_count := #current_count + 1
WHEN #current_group := CategoryID THEN #current_count := 1
END
ORDER BY CategoryID, Position -- <Column 3>, <Column 4>, ...

Answer 3

I think it would be very tedious to run additional queries all the time when you do some operations on the table. I would create a trigger that fires every time you want to insert/update something in the table.
In your case, a BEFORE UPDATE and BEFORE INSERT trigger would be advisable. If you also want to keep it clean after the deletion of an etntry, add an AFTER DELETE trigger.

Answer 4

Initial:
UPDATE content as uc
SET Position = (
SELECT count(*)
FROM content as sc
WHERE sc.CategoryId = uc.CategoryId AND sc.Position is not null)
WHERE uc.Position is null
ORDER BY uc.ContentId
Before insert:
UPDATE content
SET Position = Position+1
WHERE Position >= newPos AND CategoryId = newCat