Drupal 7: Common use cases on database updates.

mymodule.install

<?php

/**
 * Disable and uninstall a feature module.
 *
 * Note: see some edge cases where some exported components like content types
 * need to be removed manually.
 */
function mymodule_update_7100() {
  // Note that this does not take care of disabling modules that depend
  // on this one.
  module_disable(array('some_module'));
  drupal_uninstall_modules(array('some_module));
}


/**
 * Delete a field definition and all of its instances.
 */
function mymodule_update_7100(&$sandbox) {
  field_delete_field('field_name');
  field_purge_batch(1000);
}

/**
 * Delete a content type and taxonomy contained in a feature before disabling it.
 *
 * Based on http://drupal.stackexchange.com/questions/44547/unable-to-delete-feature-content-type-after-uninstalling-feature/.
 */
function mymodule_update_7100(&$sandbox) {
  $content_type = 'content_type_to_delete';
  $module = 'feature_module_where_its_exported';

  // Make content type's fields deletable.
  db_query("UPDATE {node_type} SET custom = 1, locked = 0 WHERE type = :ctype", array(':ctype' => $content_type));

  // Clean Drupal cache; this is necessary for the content type too be 'deletable'.
  drupal_flush_all_caches();
  node_type_delete($content_type);
  module_disable(array($module));
  drupal_uninstall_modules(array($module));

  // Delete taxonomies.
  $machine_names = array('exported_taxonomy1', 'exported_taxonomy2');
  foreach ($machine_names as $machine_name) {
    $taxonomy = taxonomy_vocabulary_machine_name_load($machine_name);
    taxonomy_vocabulary_delete($taxonomy->vid);
  }
}

/**
 * Drops indexes from field collection tables which are not at {field_config}
 *
 * This lets Features to recreate them and update {field_config} table
 * once database updates have completed and features are reverted.
 */
function mymodule_update_7100() {
  db_drop_index('ffield_datos_contacto', 'field_datos_contacto_revision_id');
}

/**
 * Adds a new checkbox field and sets existing nodes to default to 0.
 *
 * This is useful specially for Views and EFQ since even though you may
 * have set the default value to be 0, existing data will have a value of
 * NULL.
 */
function mymodule_update_7105() {
  // We need to revert the feature components first, to be sure the field exists
  // so that data can be saved to it.
  $items['mymodule'] = array('field_base', 'field_instance');
  features_revert($items);

  // Load up all show nodes.
  $query = new EntityFieldQuery();
  $query->entityCondition('entity_type', 'node')
    ->entityCondition('bundle', 'mycontenttype');
  $results = $query->execute();

  // If we don't have any results, throw an exception. The only reason we
  // wouldn't get any results is if something went wrong.
  if (!isset($results['node']) || empty($results['node'])) {
    $t_args = array('@function' => __FUNCTION__);
    throw new DrupalUpdateException(t('Unable to find mycontenttype nodes in update @function.', $t_args));
  }

  // Loop over each node and set field_myfield to 0.
  foreach (node_load_multiple(array_keys($results['node'])) as $node) {
    $node->field_myfield[LANGUAGE_NONE][0]['value'] = 0;
    node_save($node);
  }
}