How to: MySQL order by user_id (RAND) with pagination - php

Question

Looking for a solution to keep a random order of a user table in the database when clicking the next page button.
Actually I have a database with 1000 users and I want to display 10 users each page (in a memberlist), my query looks like this:
$sql = "SELECT * FROM users ORDER BY user_id LIMIT 1,10";
Now I would like to ORDER BY RAND() and it works, except of course when clicking the next page, then it is shuffled again and it happens sometimes that the same users will be there again.
So my question is about a solution to keep the random order I had on the first page, also on the next pages.
I thought about to set a $_SESSION variable when someone visits the memberlist for the first time with shuffled numbers from 1 to 1000 in it and then order the members by position in the $_SESSION variable where a number is equal to a user_id.
Don't know how this might be possible, but I actually imagine a solution like:
$numbers = range(1, 1000);
$shuffled_numbers = shuffle($numbers);
$sort = $_SESSION['random_user_sort'] = $shuffled_numbers;
So I will have a mysql query when clicking page two (next page) like this:
$sql = "SELECT * FROM users ORDER BY $sort LIMIT 11,20";
Any solution to let it work this way or even better ideas?

Answer 1

The RAND() function does not really generate random numbers but what's called pseudo random numbers: numbers are calculated with a deterministic formula and they're just intended to look random. To calculate a new number, you take the previous one and apply the formula to it, and that's how we get different output with a deterministic function: by using different input.
The initial number we use is known as seed. If you have a look at the manual you'll see that RAND() has an optional argument:
RAND(), RAND(N)
Returns a random floating-point value v in the range 0 <= v < 1.0. If
a constant integer argument N is specified, it is used as the seed
value, which produces a repeatable sequence of column values
You've probably figured out by now where I want to go:
mysql> SELECT language_id, name FROM language ORDER BY RAND(33);
+-------------+----------+
| language_id | name |
+-------------+----------+
| 3 | Japanese |
| 1 | English |
| 4 | Mandarin |
| 6 | German |
| 5 | French |
| 2 | Italian |
+-------------+----------+
6 rows in set (0.00 sec)
mysql> SELECT language_id, name FROM language ORDER BY RAND(33);
+-------------+----------+
| language_id | name |
+-------------+----------+
| 3 | Japanese |
| 1 | English |
| 4 | Mandarin |
| 6 | German |
| 5 | French |
| 2 | Italian |
+-------------+----------+
6 rows in set (0.00 sec)
P.S. The manual is not explicit about the seed range (it just says integer), you might need some extra research (or just some quick testing).