drupal_record_exists_snippet.php

<?php

$update = util_record_exists('table_name', $record);
drupal_write_record('table_name', $record, $update);

function util_record_exists($table, $record, $return_keys = TRUE) {
  $schema = drupal_get_schema($table);

  $sql = "SELECT COUNT({$schema['primary key'][0]}) FROM {{$table}} WHERE ";
  $keys = array();
  $params = array();
  foreach ($schema['primary key'] as $key) {
    $keys[] = $key . '=' . db_type_placeholder($schema['fields'][$key]['type']);
    $params[] = $record[$key];
  }
  $sql .= join($keys, ' AND ');

  $exists = db_result(db_query($sql, $params)) != FALSE;

  $result = $exists;
  if ($return_keys) {
    $result = $exists ? $schema['primary key'] : array();
  }
  return $result;
}