Select.php

<?php

/**
 * @file
 * Definition of Drupal\Core\Database\Query\Select
 */

namespace Drupal\Core\Database\Query;

use Drupal\Core\Database\Database;
use Drupal\Core\Database\Connection;


/**
 * Query builder for SELECT statements.
 */
class Select extends Query implements SelectInterface {

  /**
   * The fields to SELECT.
   *
   * @var array
   */
  protected $fields = array();

  /**
   * The expressions to SELECT as virtual fields.
   *
   * @var array
   */
  protected $expressions = array();

  /**
   * The tables against which to JOIN.
   *
   * This property is a nested array. Each entry is an array representing
   * a single table against which to join. The structure of each entry is:
   *
   * array(
   *   'type' => $join_type (one of INNER, LEFT OUTER, RIGHT OUTER),
   *   'table' => $table,
   *   'alias' => $alias_of_the_table,
   *   'condition' => $condition_clause_on_which_to_join,
   *   'arguments' => $array_of_arguments_for_placeholders_in_the condition.
   *   'all_fields' => TRUE to SELECT $alias.*, FALSE or NULL otherwise.
   * )
   *
   * If $table is a string, it is taken as the name of a table. If it is
   * a Select query object, it is taken as a subquery.
   *
   * @var array
   */
  protected $tables = array();

  /**
   * The fields by which to order this query.
   *
   * This is an associative array. The keys are the fields to order, and the value
   * is the direction to order, either ASC or DESC.
   *
   * @var array
   */
  protected $order = array();

  /**
   * The fields by which to group.
   *
   * @var array
   */
  protected $group = array();