Mysql Limit rows by field value - php

Question

I'm working on simple application using PHP and MySQL. Up to this point we needed to display items from database in HTML table. Simple pagination was implemented as well. It looks something like this:
+----+---------------------+
| 1 | Item 1 |
+----+---------------------+
| 2 | Item 2 |
+----+---------------------+
| 3 | Item 3 |
+----+---------------------+
| 4 | Item 4 |
+----+---------------------+
....
+----+---------------------+
| 5 | Item 25 |
+----+---------------------+
Not a rocket science. Now we add new functionality so we can (optionally) group items - We really create a 'lot' of identical items. We decided to add new column in database called groupID - which can be number or NULL for items not contained in any group. On web page we must display it as one element which expands when you click on it.
+----+---------------------+
| 1 | Item 1 |
+----+---------------------+
| 2 | Item 2 |
+----+---------------------+
| 3 | Item 3 |
+----+---------------------+
| 4 | Group 1 (Expanded) |
+----+---------------------+
| Group 1 Item 1 |
+---------------------+
| Group 1 Item 2 |
+---------------------+
| Group 1 Item 3 |
+----+---------------------+
....
+----+---------------------+
| 25| Item 25 |
+----+---------------------+
As you can see Number of items on one page may vary so we must treat items in group as one item, so simple 'limit 25' not working anymore. I wonder if I can make some clever mysql query which will work this way. I rather want to avoid to create new table in database which consists groups and relation to item table, because most of the groups will have only 1 Item. I don't believe this functionality will be used a lot, but You know - client. Also this system works on production for some time so I'd rather avoid such changes. Any Idea how to make it work? Also please keep it simple as possible, because this example is simplified. Real query is already bit complicated.
I also want avoid parsing it via PHP code, because it's just dumb to query all few thousands of rows and then discard all but 25-50 elements.

Answer 1

As you said some group ids can be null, I thought we should fix that.
When null, we use the item id to for the group and we use a prefix to make sure our new group_id is unique.
This is my solution using subqueries (not pretty, but seems to work):
SELECT
i1.id,
i1.itemgroup1
FROM (
SELECT
items.id,
IF(ISNULL(items.group_id),
CONCAT('alone-', items.id),
CONCAT('group-', items.group_id)) as itemgroup1
FROM
items
) as i1
RIGHT JOIN (
SELECT
items.id,
IF(ISNULL(items.group_id),
CONCAT('alone-', items.id),
CONCAT('group-', items.group_id)) as itemgroup2
FROM
items
GROUP BY itemgroup2
LIMIT 2
) as i2 on i2.itemgroup2 = i1.itemgroup1
** UPDATE **
Removed
WHERE
items.group_id IS NOT NULL