SQL Selecting “Window” Around Particular Row

Question

It\'s quite possible a question like this has been asked before, but I can\'t think of the terms to search for.

I\'m working on a photo gallery application, and want

Answer 1

This is standard "Row ordering" problem... If your database has rowId capability you can use that, otherwise you need a subquery that counts the numnber of rows with Ids less than the id of the current row... like this:

-- asssuming @Id is value of id in the "middle"

 Select *  From Photos P
 Where (Select Count(*) From Photos
         Where id <= P.Id)
     Between (Select Count(*) From Photos
              Where id < @Id) - 4
        And  (Select Count(*) From Photos
              Where id < @Id) + 4

As a comment raised the album issue you would want to add album predicate to each subquery

   Select *  From Photos P
   Where (Select Count(*) From Photos
          Where album = @album
            And id <= P.Id)
     Between (Select Case When Count(*) < 4 
                      Then 4 Else Count(*) End
              From Photos
              Where album = @album
                 And id < @Id) - 4
        And  (Select Case When Count(*) < 4 
                      Then 4 Else Count(*) End
              From Photos
              Where album = @album
                  And id < @Id) + 4

Answer 2

Probably could just use a UNION, and then trim off the extra results in the procedural code that displays the results (as this will return 20 rows in the non-edge cases):

(SELECT 
     * 
FROM photos
   WHERE ID < #current_id#
   ORDER BY ID DESC LIMIT 10)
UNION
  (SELECT *
   FROM photos
   WHERE ID >= #current_id#
   ORDER BY ID ASC LIMIT 10)
ORDER BY ID ASC

EDIT: Increased limit to 10 on both sides of the UNION, as suggested by le dorfier.

EDIT 2: Modified to better reflect final implementation, as suggested by Dominic.

Answer 3

If you are using SQL Server, you can use the row_number() function to give you the row order index and do something like this:

declare @selected_photo integer;
set @selected_photo = 5;

declare @buffer_size integer;
set @buffer_size = 2;

select
   ph.rownum,
   ph.id
from
   (select row_number() over (order by Id) as rownum, * from Photos) as ph
where
   ph.rownum between case
                         when @selected_photo - @buffer_size < 1 then 1
                         else @selected_photo - @buffer_size
                      end
                      and @selected_photo + @buffer_size

Edit: Here is an article on simulating the row_number() function in MySQL, combining that with this might get you what you need - I'd try it but don't have a MySQL db handy to play with at work. :-)

http://www.xaprb.com/blog/2006/12/02/how-to-number-rows-in-mysql/