<?php
/**
* Demonstration of weighted random selection of MySQL database.
*/
$conn = mysql_connect('localhost', 'root', '');
// prepare table and data.
mysql_select_db('test', $conn);
mysql_query("drop table if exists temp_wrs", $conn);
mysql_query("create table temp_wrs (
id int not null auto_increment,
val varchar(16),
weight tinyint,
upto smallint,
primary key (id)
)", $conn);
$base_data = array( // value-weight pair array.
'A' => 5,
'B' => 3,
'C' => 2,
'D' => 7,
'E' => 6,
'F' => 3,
'G' => 5,
'H' => 4
);
foreach($base_data as $val => $weight) {
mysql_query("insert into temp_wrs (val, weight) values ('".$val."', ".$weight.")", $conn);
}
// calculate the sum of weight.
$rs = mysql_query('select sum(weight) as s from temp_wrs', $conn);
$row = mysql_fetch_assoc($rs);
$sum = $row['s'];
mysql_free_result($rs);
// update range based on their weight.
// each "upto" columns will set by sub-sum of weight.
mysql_query("update temp_wrs a, (
select id, (select sum(weight) from temp_wrs where id <= i.id) as subsum from temp_wrs i
) b
set a.upto = b.subsum
where a.id = b.id", $conn);
$result = array();
foreach($base_data as $val => $weight) {
$result[$val] = 0;
}
// do weighted random select ($sum * $times) times.
$times = 100;
$loop_count = $sum * $times;
for($i = 0; $i < $loop_count; $i++) {
$rand = rand(0, $sum-1);
// select the row which $rand pointing.
$rs = mysql_query('select * from temp_wrs where upto > '.$rand.' order by id limit 1', $conn);
$row = mysql_fetch_assoc($rs);
$result[$row['val']] += 1;
mysql_free_result($rs);
}
// clean up.
mysql_query("drop table if exists temp_wrs");
mysql_close($conn);
?>
<table>
<thead>
<th>DATA</th>
<th>WEIGHT</th>
<th>ACTUALLY SELECTED<br />BY <?php echo $loop_count; ?> TIMES</th>
</thead>
<tbody>
<?php foreach($base_data as $val => $weight) : ?>
<tr>
<th><?php echo $val; ?></th>
<td><?php echo $weight; ?></td>
<td><?php echo $result[$val]; ?></td>
</tr>
<?php endforeach; ?>
<tbody>
</table>
如果要选择 N 行...
- 重新计算总和。
- 重置范围(“upto”列)。
- 选择
$rand
指向的行。
应在每个选择循环中排除先前选择的行。where ... id not in (3, 5);