gistfile1.php

<?php

/**
 * @file
 * flag/flag.views.inc
 * /

/**
 * Implements hook_views_data().
 * /
function flag_views_data() {
  $data = array();

  // Add the {flagging} table data to a 'Flag' group in the Views UI.
  $data['flagging']['table']['group'] = t('Flag');

  // Define how the {flagging} table should join with {node}.
  // Note: this is ignoring {flagging}.entity_type and should not.
  $data['flagging']['table']['join'] = array(
    'node' => array(
      'type' => 'INNER',
      'left_field' => 'nid',
      'field' =>  'entity_id',
    ),
  );

  // Expose the {flagging}.entity_id field.
  $data['flagging']['entity_id'] = array(
    'title' => t('Entity ID'),
    'help' => t('The ID of the entity'),
    // Add a sort criterion based on the entity_id field.
    'sort' => array(
      'id' => 'standard',
    ),
  );

  return $data;
}

Note that this is not at all sufficient to integrate Flag with node views since it's ignoring the {flagging}.entity_type field, plus a sort criterion for nodes based on {flagging}.entity_id is silly anyway. It just demonstrates that the hook_views_data() is being parsed by Views and generating a valid SQL query with the table and field. :)