Adding multiple new fields to a table

mymodule.install.php

<?php
// @see: https://www.drupal.org/node/150215

/**
 * Add new fields to 'mytable' table.
 */
function MYMODULE_update_7001() {
  $fields = array(
    'field_name_1' => array(
      'description' => 'Field name 1',
      'type' => 'varchar',
      'length' => '32',
      'not null' => FALSE,
      'default' => '',
    ),
    'field_name_2'=> array(
      'description' => 'Field name 2',
      'type' => 'varchar',
      'length' => '32',
      'not null' => FALSE,
      'default' => '',
    ),
    'field_name_3' => array(
      'description' => 'Field name 3',
      'type' => 'varchar',
      'length' => '128',
      'not null' => FALSE,
      'default' => '',
    ),
  );

  foreach ($fields as $key => $field)  {
    if (!db_field_exists('mytable', $key)) {
      db_add_field('mytable', $key, $field);
    }
  }
}
?>