How to distribute records based on column - mysql

Question

I have a table storing competition entries.
Contestants must enter text and can optionally upload a photo
When I display entries on a page I paginate through 9 at a time.
How can I ensure as much as possible that each page contains at least one entry with a photo in it (presuming there is enough photo entries for one per page)? It would probably be sufficient to distribute entries with photos evenly amongst the pages

Answer 1

This was one of the more challenging questions I've seen recently -- thanks for that! I'm not able to get it working using a single SQL statement, but I was able to get it working (at least it appears) like this. Basically it tries to determine how many results will be returned, then how many of those have photos, and uses a percentage of the photos divided by the number of pages (using CEILING to ensure at least one for the first few pages).
Anyhow, here goes:
SET #page = 1;
SET #resultsPerPage = 9;
SELECT #recCount:= COUNT(Id) as RecCount
FROM Entries;
SELECT #photoCount:= COUNT(Photo) as PhotoCount
FROM Entries
WHERE Photo IS NOT NULL;
SET #pageCount = CEILING(#recCount/#resultsPerPage);
SET #photosPerPage = CEILING(#photoCount/#pageCount);
SET #nonPhotosPerPage = #resultsPerPage - CEILING(#photosPerPage);
SELECT *
FROM (
SELECT *,
#rownum := #rownum + 1 row_number
FROM Entries JOIN (SELECT #rownum := 0) r
WHERE Photo IS NOT NULL
) a
WHERE a.row_number > (#photosPerPage*(#page-1))
and a.row_number <= (#photosPerPage*(#page))
UNION
SELECT *
FROM (
SELECT *,
#rownum2 := #rownum2 + 1 row_number
FROM Entries JOIN (SELECT #rownum2 := 0) r
WHERE Photo IS NULL
) b
WHERE b.row_number > (#nonPhotosPerPage*(#page-1))
and b.row_number <= (#nonPhotosPerPage*(#page))
And the SQL Fiddle.
Best of luck!

Answer 2

I would suggest that you randomly order the rows:
order by rand()
This doesn't guarantee a photo on every page, but it helps.
The alternative is to do something like this:
select *, #seqnum:=#seqnum+1
from t
where nophoto
select *, #seqnum:=#seqnum+8
from t
where photo
Then sort by seqnum. What makes this cumbersome is handling the cases where there is fewer than one photo per page and more than one photo. The random method is probably sufficient.

Answer 3

For each page, do this (eg for page 3, page size 10):
select ...
from ...
where has_photo
order by created
limit 3, 1
union
select ...
from ...
where not has_photo
order by created
limit 27, 9
This query breaks up the two types of rows into two separate queries recombined by union.