MySQL: what's the most efficient way to select multiple random rows

Question

I have a fairly large dataset and a query that requires two joins, so efficiency of the query is very important to me. I need to retrieve 3 random rows from the database tha

Answer 1

Adding your RAND() call into the ORDER BY clause should allow you to ignore the ID. Try this:

SELECT * FROM table WHERE ... ORDER BY RAND() LIMIT 3;

After having performance issues pointed out, your best bet may be something along these lines (utilizing PHP):

$result = PDO:query('SELECT MAX(id) FROM table');
$max    = $result->fetchColumn();
$ids    = array();
$rows   = 5;

for ($i = 0; $i < $rows; $i++) {
    $ids[] = rand(1, $max);
}

$ids     = implode(', ', $ids);
$query   = PDO::prepare('SELECT * FROM table WHERE id IN (:ids)');
$results = $query->execute(array('ids' => $ids));

At this point you should be able to select the first 3 results. The only issue with this approach is dealing with deleted rows and you might have to either bump the $rows var or add some logic to do another query in case you didn't receive at least 3 results back.