Delete the values with the oldest timestamp

Question

I want when cron runs to delete the oldest values of timestamp and in same time create the new one. I have this code for cron. Here select all the values from watchdog:

function error_cron() {

// Begin building the query.
$query = db_select('watchdog', 'th')
  ->extend('PagerDefault')
  ->orderBy('wid')
  ->fields('th', array('variables', 'type', 'severity',
       'message', 'wid', 'timestamp'))
  ->limit(2000);

// Fetch the result set.
$result = $query -> execute();

// Loop through each item and add to $row.
foreach ($result as $row) {
  error_table($row);
}

In the next code i create the distinct values of the watchdog in my table:

function error_table($row) {

  $variables = $row -> variables;
  $timestamp = $row -> timestamp;
  $wid = $row -> wid;

  $r = db_select('error', 'b')
    -> fields('b')
    -> condition('variables', $variables, '=')
    -> condition('timestamp', $timestamp, '<=')
    -> execute();

  if($r -> rowCount() == 0) {
    $query = db_insert('error')
      ->fields(array(
        'timestamp' => $timestamp,
        'wid' => $wid,
        'variables' => $variables,
     ))
     ->execute();
  }
}

Ok with all this code the table has been created and it has the distinct values but when cron reruns it shows the same values.. I want when there is a row with same 'variables' to delete the row with the oldest timestamp and create the new one with the newest timestamp.

Answer 1

I solve my problem with this lines of code:

$giannis = db_delete('error')
    -> condition('variables', $variables, '=')
    -> condition('timestamp', $timestamp, '<')
    ->execute();