Btukfyl

Have a table photos

photos.id

photos.user_id

photos.order

A) Is it possible via a single query to group all photos by user and then update the order 1,2,3..N ?

B) added twist, what if some of the photos already have an order value associated? Make sure that the new photos.order never gets repeated and fills in ant orders lower or higher than those existing (as best as possible)

My only thought is just to run a script on this and loop through it and re'order' everything?

photos.id int(10)

photos.created_at datetime

photos.order int(10)

photos.user_id int(10)

Right now data may look like this

user_id = 1

photo_id = 1

order = NULL



user_id = 2

photo_id = 2

order = NULL



user_id = 1

photo_id = 3

order = NULL

the desired result would be

user_id = 1

photo_id = 1

order = 1



user_id = 2

photo_id = 2

order = 1



user_id = 1

photo_id = 3

order = 2

A)

You can use a variable that increments with each row and resets with each user_ID to get the row count.

SELECT  ID,

        User_ID,

        `Order`

FROM    (   SELECT  @r:= IF(@u = User_ID, @r + 1,1) AS `Order`,

                    ID,

                    User_ID,

                    @u:= User_ID

            FROM    Photos,

                    (SELECT @r:= 1) AS r,

                    (SELECT @u:= 0) AS u

            ORDER BY User_ID, ID

        ) AS Photos

Example on SQL Fiddle

B)

My First solution was to just add Order to the sorting that adds the row number, therefore anything with an Order Gets sorted by its order first, but this only works if your ordering system has no gaps and starts at 1:

SELECT  ID,

        User_ID,

        RowNumber AS `Order`

FROM    (   SELECT  @r:= IF(@u = User_ID, @r + 1,1) AS `RowNumber`,

                    ID,

                    User_ID,

                    @u:= User_ID

            FROM    Photos,

                    (SELECT @i:= 1) AS r,

                    (SELECT @u:= 0) AS u

            ORDER BY User_ID, `Order`, ID

        ) AS Photos

ORDER BY `User_ID`, `Order`

Example using Order Field

ORDERING WITH GAPS

I have eventually found a way of maintaining the sort order even when there are gaps in the sequence.

SELECT  ID, User_ID, `Order`

FROM    Photos

WHERE   `Order` IS NOT NULL

UNION ALL

SELECT  Photos.ID,

        Photos.user_ID,

        Numbers.RowNum

FROM    (   SELECT  ID,

                    User_ID,

                    @r1:= IF(@u1 = User_ID,@r1 + 1,1) AS RowNum,

                    @u1:= User_ID 

            FROM    Photos,

                    (SELECT @r1:= 0) AS r,

                    (SELECT @u1:= 0) AS u

            WHERE   `Order` IS NULL

            ORDER BY User_ID, ID

        ) AS Photos

        INNER JOIN

        (   SELECT  User_ID,

                    RowNum,

                    @r2:= IF(@u2 = User_ID,@r2 + 1,1) AS RowNum2,

                    @u2:= User_ID 

            FROM    (   SELECT  DISTINCT p.User_ID, o.RowNum

                        FROM    Photos AS p,

                                (   SELECT  @i:= @i + 1 AS RowNum

                                    FROM    INFORMATION_SCHEMA.COLLATION_CHARACTER_SET_APPLICABILITY,

                                            ( SELECT @i:= 0) AS i

                                ) AS o

                        WHERE   RowNum <= (SELECT COUNT(*) FROM Photos P1 WHERE p.User_ID = p1.User_ID)

                        AND     NOT EXISTS

                                (   SELECT  1

                                    FROM    Photos p2

                                    WHERE   p.User_ID = p2.User_ID

                                    AND     o.RowNum = p2.`Order`

                                )

                        AND     p.`Order` IS NULL

                        ORDER BY User_ID, RowNum

                    ) AS p,

                    (SELECT @r2:= 0) AS r,

                    (SELECT @u2:= 0) AS u

            ORDER BY user_ID, RowNum

        ) AS numbers

            ON Photos.User_ID = numbers.User_ID

            AND photos.RowNum = numbers.RowNum2

ORDER BY User_ID, `Order`

However as you can see this is pretty complicated. This works by treating those with an order value separately to those without. The top query just ranks all photos with no order value in order of ID for each user. The bottom query uses a cross join to generates a sequential list from 1 to n for each user ID (up to the number of entries for each User_ID). So with a data set like this:

ID  User_ID Order

1   1       NULL

2   2       NULL

3   1       NULL

4   1       1

5   1       3

6   2       2

7   2       3

It would generate

UserID  RowNum

1       1

1       2

1       3

1       4

2       1

2       2

2       3

It then uses NOT EXISTS to elimiate all combinations already used by Photos with a non null order, and ranked in order of RowNum partitioned by User_ID giving

UserID  RowNum  Rownum2

1       2       1

1       4       2

2       1       1

The RowNum2 value can then be matched with the rownum value achieved in the from subquery, giving the correct order value. Long winded, but it works.

Example on SQL Fiddle

Worked for me. I needed to increment version grouping by 4 fields (host, folder, fileName, status) and sort by 1 (downloadedAtTicks).
This is is my SELECT

SET @status := NULL;

SET @version := NULL;

SELECT

  id,

  host,

  folder,

  fileName,

  status,

  downloadedAtTicks,

  version,

  IF(IF(status IS NULL, 0, status) = @status, @version := @version + 1, @version := 0) AS varVersion,

  @status := IF(status IS NULL, 0, status) AS varStatus

FROM csvsource

ORDER BY host, folder, fileName, status, downloadedAtTicks;

And this is my UPDATE

SET @status := NULL;

SET @version := NULL;

UPDATE

    csvsource csv,

    (SELECT

       id,

       IF(IF(status IS NULL, 0, status) = @status, @version := @version + 1, @version := 0) AS varVersion,

       @status := IF(status IS NULL, 0, status) AS varStatus

     FROM csvsource

     ORDER BY host, folder, fileName, status, downloadedAtTicks) AS sub

SET

  csv.version = sub.varVersion

WHERE csv.id = sub.id;