Custom MySQL Ordering - php

Question

I have a query that needs a custom sorting, trimmed down to the bare minimums something like:
SELECT u.*, p.*, p.id as product_id
FROM users u, products p
WHERE u.id = p.user_id
ORDER BY product_id DESC
And I get returned a set of rows like:
UserID ProductID
2 5
2 4
3 3
1 2
1 1
But I want it to actually sort SOMETHING like this (so no 2 UserIDs are adjacent to eachother):
UserID ProductID
1 2
2 4
3 3
2 5
1 1
Is this even possible with MySQL, or do I need some PHP magic?

Answer 1

A canonical way of solving this problem is by enumerating the duplicate rows and then ordering by that value:
select t.*
from (SELECT u.*, p.*, p.id as product_id,
row_number() over (partition by u.id order by (select NULL)) as seqnum
FROM users u join
products p
on u.id = p.user_id
) t
order by seqnum, id;
This will work, as long as no one user has a really long sequence (as in your example).
There is no "always-works" solution, because it is easy to come up with a situation where your goal is not possible.

Answer 2

Here fetch your sorted results into an array. Then do something like this.
$records = $res->fetchAll();
$count = count($records);
$records = array_chunk($records, ceil(count($records)/2);
$unsorted = array();
for($x = 0; $x < $count; $x++){
$unsorted[] = $records[$x%2][floor($x/2)];
}

Answer 3

Consider the following...
CREATE TABLE sortable(id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,player_id INT NOT NULL);
INSERT INTO sortable(player_id) VALUES (1),(1),(2),(3),(4),(3),(3),(2),(1),(2),(4),(4);
SELECT * FROM sortable;
+----+-----------+
| id | player_id |
+----+-----------+
| 1 | 1 |
| 2 | 1 |
| 3 | 2 |
| 4 | 3 |
| 5 | 4 |
| 6 | 3 |
| 7 | 3 |
| 8 | 2 |
| 9 | 1 |
| 10 | 2 |
| 11 | 4 |
| 12 | 4 |
+----+-----------+
SELECT x.*,COUNT(*) rank FROM sortable x JOIn sortable y ON y.player_id = x.player_id AND y.id <= x.id GROUP BY x.id ORDER BY player_id,rank;
+----+-----------+------+
| id | player_id | rank |
+----+-----------+------+
| 1 | 1 | 1 |
| 2 | 1 | 2 |
| 9 | 1 | 3 |
| 3 | 2 | 1 |
| 8 | 2 | 2 |
| 10 | 2 | 3 |
| 4 | 3 | 1 |
| 6 | 3 | 2 |
| 7 | 3 | 3 |
| 5 | 4 | 1 |
| 11 | 4 | 2 |
| 12 | 4 | 3 |
+----+-----------+------+
SELECT x.*,COUNT(*) rank FROM sortable x JOIn sortable y ON y.player_id = x.player_id AND y.id <= x.id GROUP BY x.id ORDER BY rank;
+----+-----------+------+
| id | player_id | rank |
+----+-----------+------+
| 1 | 1 | 1 |
| 3 | 2 | 1 |
| 4 | 3 | 1 |
| 5 | 4 | 1 |
| 2 | 1 | 2 |
| 8 | 2 | 2 |
| 6 | 3 | 2 |
| 11 | 4 | 2 |
| 9 | 1 | 3 |
| 10 | 2 | 3 |
| 7 | 3 | 3 |
| 12 | 4 | 3 |
+----+-----------+------+

Answer 4

So if your problem is just that you dont want two records with same id should not come next to each other wha I can think simplest is use
SELECT u.*, p.*, p.id as product_id
FROM users u, products p
WHERE u.id = p.user_id
ORDER BY user_id%2 DESC
Or you can even use other number than 2 to meet any certain order you want....