Selecting random rows with MySQL

Question

I saw many topics about this subject and I have been unsuccessful in understanding how to do it.

For example, if I have this table:

+------+-------+-------+
| id   | name  | class |
+------+-------+-------+
|    5 | test  | one   | 
|   10 | test2 | one   | 
|   12 | test5 | one   | 
|    7 | test6 | two   | 
+------+-------+-------+

and I want to show only X random rows from class "one", how can I do that?

NOTE: it's a big table, so I don't want to use ORDER BY RAND.

Answer 1

The ORDER BY RAND() solution that most people recommend doesn't scale to large tables, as you already know.

SET @r := (SELECT FLOOR(RAND() * (SELECT COUNT(*) FROM mytable)));
SET @sql := CONCAT('SELECT * FROM mytable LIMIT 1 OFFSET ', @r);
PREPARE stmt1 FROM @sql;
EXECUTE stmt1;

I cover this and other solutions in my book, SQL Antipatterns: Avoiding the Pitfalls of Database Programming.

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If you want to do this with PHP, you could do something like this (not tested):

<?php
$mysqli->begin_transaction();
$result = $mysqli->query("SELECT COUNT(*) FROM mytable")
$row = $result->fetch_row(); 
$count = $row[0]; 
$offset = mt_rand(0, $count);
$result = $mysqli->query("SELECT * FROM mytable LIMIT 1 OFFSET $offset");
...
$mysqli->commit();