Example of using drupal_write_record over a conditional db_insert and db_update.

drupal_write_record_example.php

<?php

// Let's say for argument sake $query is a database query that checks for the 
// presence of an existing record and returns FALSE if nothing is found. In this 
// example, we need to check for an existing record and update the row if it 
// exists or create a new entry if it doesn't.

/**
 * Bad
 */
if ($query == FALSE) {
  $entry = db_insert('my_custom_module_table')
    ->fields(array(
      'field_a' => 'one value',
      'field_b' => 'another value',
      'created' => REQUEST_TIME,
      'changed' => REQUEST_TIME,
    ))
    ->execute();
}
else {
  $entry = db_update('my_custom_module_table')
    ->fields(array(
      'field_a' => 'one value',
      'field_b' => 'another value',
      'changed' => REQUEST_TIME,
    ))
    ->condition('uid', $query->uid, '=')
    ->range(0, 1)
    ->execute();
}

/**
 * Good
 */
$entry_data = array(
  'field_a' => 'one value',
  'field_b' => 'another value',
  'changed' => REQUEST_TIME,
);

if ($query == FALSE) {
  $entry_data['created'] = REQUEST_TIME;
  drupal_write_record('my_custom_module_table', $entry_data);
}
else {
  drupal_write_record('my_custom_module_table', $entry_data, array('uid'));
}