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    <p>You can either do this in PHP as you do or by using a MySQL trigger.</p> <p>If you stick to the PHP way, then instead of generating a number every time, do something like this</p> <pre><code>$found = 0; $cityarr = array(); $numberarr = array(); //create the cityarr for($i=1; $i&lt;=10;$i++) $cityarr[] = i; //create the numberarr for($i=1; $i&lt;=250;$i++) $numberarr[] = i; //shuffle the arrays shuffle($cityarr); shuffle($numberarr); //iterate until you find n unused one foreach($cityarr as $city) { foreach($numberarr as $num) { $sql_result2 = mysql_query("SELECT * FROM houses WHERE city='$city' AND number='$num'", $db); if (mysql_num_rows($sql_result2) == 0) { $found = 1; break; } } if($found) break; } </code></pre> <p>this way you don't check the same value more than once, and you still check randomly.</p> <p>But you should really consider fetching all your records before the loops, so you only have one query. That would also increase the performance a lot.</p> <p>like</p> <pre><code>$taken = array(); for($i=1; $i&lt;=10;$i++) $taken[i] = array(); $records = mysql_query("SELECT * FROM houses", $db); while($rec = mysql_fetch_assoc($records)) { $taken[$rec['city']][] = $rec['number']; } for($i=1; $i&lt;=10;$i++) $cityarr[] = i; for($i=1; $i&lt;=250;$i++) $numberarr[] = i; foreach($cityarr as $city) { foreach($numberarr as $num) { if(in_array($num, $taken[]) { $cityNotTaken = $city; $numberNotTaken = $number; $found = 1; break; } } if($found) break; } echo 'City ' . $cityNotTaken . ' number ' . $numberNotTaken . ' is not taken!'; </code></pre> <p>I would go with this method :-)</p>
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