Dominic Rodger Dominic Rodger - 4 months ago 8
SQL Question

SQL Selecting "Window" Around Particular Row

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 to display 9 thumbnails showing the context of the current photo being shown (in a 3x3 grid with the current photo in the centre, unless the current photo is in the first 4 photos being shown, in which case if e.g. if the current photo is the 2nd I want to select photos 1 through 9). For example, given an album containing the list of photos with ids:

1, 5, 9, 12, 13, 18, 19, 20, 21, 22, 23, 25, 26

If the current photo is 19, I want to also view:

9, 12, 13, 18, 19, 20, 21, 22, 23

If the current photo is 5, I want to also view:

1, 5, 9, 12, 13, 18, 19, 20, 21

I've been thinking of something along the lines of:

SELECT *
FROM photos
WHERE ABS(id - currentphoto) < 5
ORDER BY id ASC
LIMIT 25


but this doesn't work in the case where the ids are non-sequential (as in the example above), or for the case where there are insufficient photos before the currentphoto.

Any thoughts?

Thanks,

Dom

p.s. Please leave a comment if anything is unclear, and I'll clarify the question. If anyone can think of a more useful title to help other people find this question in future, then please comment too.

Answer

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.