select.inc

<?php

/**
 * @ingroup database
 * @{
 */

require_once __DIR__ . '/query.inc';

/**
 * Interface for extendable query objects.
 *
 * "Extenders" follow the "Decorator" OOP design pattern.  That is, they wrap
 * and "decorate" another object.  In our case, they implement the same interface
 * as select queries and wrap a select query, to which they delegate almost all
 * operations.  Subclasses of this class may implement additional methods or
 * override existing methods as appropriate.  Extenders may also wrap other
 * extender objects, allowing for arbitrarily complex "enhanced" queries.
 */
interface QueryExtendableInterface {

  /**
   * Enhance this object by wrapping it in an extender object.
   *
   * @param $extender_name
   *   The base name of the extending class.  The base name will be checked
   *   against the current database connection to allow driver-specific subclasses
   *   as well, using the same logic as the query objects themselves.  For example,
   *   PagerDefault_mysql is the MySQL-specific override for PagerDefault.
   * @return QueryExtendableInterface
   *   The extender object, which now contains a reference to this object.
   */
  public function extend($extender_name);
}

/**
 * Interface definition for a Select Query object.
 */
interface SelectQueryInterface extends QueryConditionInterface, QueryAlterableInterface, QueryExtendableInterface, QueryPlaceholderInterface {

  /* Alter accessors to expose the query data to alter hooks. */

  /**
   * Returns a reference to the fields array for this query.
   *
   * Because this method returns by reference, alter hooks may edit the fields
   * array directly to make their changes. If just adding fields, however, the
   * use of addField() is preferred.
   *
   * Note that this method must be called by reference as well:
   *
   * @code
   * $fields =& $query->getFields();
   * @endcode
   *
   * @return
   *   A reference to the fields array structure.
   */
  public function &getFields();

  /**
   * Returns a reference to the expressions array for this query.
   *
   * Because this method returns by reference, alter hooks may edit the expressions
   * array directly to make their changes. If just adding expressions, however, the
   * use of addExpression() is preferred.
   *
   * Note that this method must be called by reference as well:
   *
   * @code
   * $fields =& $query->getExpressions();
   * @endcode
   *
   * @return
   *   A reference to the expression array structure.
   */
  public function &getExpressions();

  /**
   * Returns a reference to the order by array for this query.
   *
   * Because this method returns by reference, alter hooks may edit the order-by
   * array directly to make their changes. If just adding additional ordering
   * fields, however, the use of orderBy() is preferred.
   *
   * Note that this method must be called by reference as well:
   *
   * @code
   * $fields =& $query->getOrderBy();
   * @endcode
   *
   * @return
   *   A reference to the expression array structure.
   */
  public function &getOrderBy();

  /**
   * Returns a reference to the group-by array for this query.
   *
   * Because this method returns by reference, alter hooks may edit the group-by
   * array directly to make their changes. If just adding additional grouping
   * fields, however, the use of groupBy() is preferred.
   *
   * Note that this method must be called by reference as well:
   *
   * @code
   * $fields =& $query->getGroupBy();
   * @endcode
   *
   * @return
   *   A reference to the group-by array structure.
   */
  public function &getGroupBy();

  /**
   * Returns a reference to the tables array for this query.
   *
   * Because this method returns by reference, alter hooks may edit the tables
   * array directly to make their changes. If just adding tables, however, the
   * use of the join() methods is preferred.
   *
   * Note that this method must be called by reference as well:
   *
   * @code
   * $fields =& $query->getTables();
   * @endcode
   *
   * @return
   *   A reference to the tables array structure.
   */
  public function &getTables();

  /**
   * Returns a reference to the union queries for this query. This include
   * queries for UNION, UNION ALL, and UNION DISTINCT.
   *
   * Because this method returns by reference, alter hooks may edit the tables
   * array directly to make their changes. If just adding union queries,
   * however, the use of the union() method is preferred.
   *
   * Note that this method must be called by reference as well:
   *
   * @code
   * $fields =& $query->getUnion();
   * @endcode
   *
   * @return
   *   A reference to the union query array structure.
   */
  public function &getUnion();

  /**
   * Compiles and returns an associative array of the arguments for this prepared statement.
   *
   * @param $queryPlaceholder
   *   When collecting the arguments of a subquery, the main placeholder
   *   object should be passed as this parameter.
   *
   * @return
   *   An associative array of all placeholder arguments for this query.
   */
  public function getArguments(QueryPlaceholderInterface $queryPlaceholder = NULL);

  /* Query building operations */

  /**
   * Sets this query to be DISTINCT.
   *
   * @param $distinct
   *   TRUE to flag this query DISTINCT, FALSE to disable it.
   * @return SelectQueryInterface
   *   The called object.
   */
  public function distinct($distinct = TRUE);

  /**
   * Adds a field to the list to be SELECTed.
   *
   * @param $table_alias
   *   The name of the table from which the field comes, as an alias. Generally
   *   you will want to use the return value of join() here to ensure that it is
   *   valid.
   * @param $field
   *   The name of the field.
   * @param $alias
   *   The alias for this field. If not specified, one will be generated
   *   automatically based on the $table_alias and $field. The alias will be
   *   checked for uniqueness, so the requested alias may not be the alias
   *   that is assigned in all cases.
   * @return
   *   The unique alias that was assigned for this field.
   */
  public function addField($table_alias, $field, $alias = NULL);

  /**
   * Add multiple fields from the same table to be SELECTed.
   *
   * This method does not return the aliases set for the passed fields. In the
   * majority of cases that is not a problem, as the alias will be the field
   * name. However, if you do need to know the alias you can call getFields()
   * and examine the result to determine what alias was created. Alternatively,
   * simply use addField() for the few fields you care about and this method for
   * the rest.
   *
   * @param $table_alias
   *   The name of the table from which the field comes, as an alias. Generally
   *   you will want to use the return value of join() here to ensure that it is
   *   valid.
   * @param $fields
   *   An indexed array of fields present in the specified table that should be
   *   included in this query. If not specified, $table_alias.* will be generated
   *   without any aliases.
   * @return SelectQueryInterface
   *   The called object.
   */
  public function fields($table_alias, array $fields = array());

  /**
   * Adds an expression to the list of "fields" to be SELECTed.
   *
   * An expression can be any arbitrary string that is valid SQL. That includes
   * various functions, which may in some cases be database-dependent. This
   * method makes no effort to correct for database-specific functions.
   *
   * @param $expression
   *   The expression string. May contain placeholders.
   * @param $alias
   *   The alias for this expression. If not specified, one will be generated
   *   automatically in the form "expression_#". The alias will be checked for
   *   uniqueness, so the requested alias may not be the alias that is assigned
   *   in all cases.
   * @param $arguments
   *   Any placeholder arguments needed for this expression.
   * @return
   *   The unique alias that was assigned for this expression.
   */
  public function addExpression($expression, $alias = NULL, $arguments = array());

  /**
   * Default Join against another table in the database.
   *
   * This method is a convenience method for innerJoin().
   *
   * @param $table
   *   The table against which to join.
   * @param $alias
   *   The alias for the table. In most cases this should be the first letter
   *   of the table, or the first letter of each "word" in the table.
   * @param $condition
   *   The condition on which to join this table. If the join requires values,
   *   this clause should use a named placeholder and the value or values to
   *   insert should be passed in the 4th parameter. For the first table joined
   *   on a query, this value is ignored as the first table is taken as the base
   *   table. The token %alias can be used in this string to be replaced with
   *   the actual alias. This is useful when $alias is modified by the database
   *   system, for example, when joining the same table more than once.
   * @param $arguments
   *   An array of arguments to replace into the $condition of this join.
   * @return
   *   The unique alias that was assigned for this table.
   */
  public function join($table, $alias = NULL, $condition = NULL, $arguments = array());

  /**
   * Inner Join against another table in the database.
   *
   * @param $table
   *   The table against which to join.
   * @param $alias
   *   The alias for the table. In most cases this should be the first letter
   *   of the table, or the first letter of each "word" in the table.
   * @param $condition
   *   The condition on which to join this table. If the join requires values,
   *   this clause should use a named placeholder and the value or values to
   *   insert should be passed in the 4th parameter. For the first table joined
   *   on a query, this value is ignored as the first table is taken as the base
   *   table. The token %alias can be used in this string to be replaced with
   *   the actual alias. This is useful when $alias is modified by the database
   *   system, for example, when joining the same table more than once.
   * @param $arguments
   *   An array of arguments to replace into the $condition of this join.
   * @return
   *   The unique alias that was assigned for this table.
   */
  public function innerJoin($table, $alias = NULL, $condition = NULL, $arguments = array());

  /**
   * Left Outer Join against another table in the database.
   *
   * @param $table
   *   The table against which to join.
   * @param $alias
   *   The alias for the table. In most cases this should be the first letter
   *   of the table, or the first letter of each "word" in the table.
   * @param $condition
   *   The condition on which to join this table. If the join requires values,
   *   this clause should use a named placeholder and the value or values to
   *   insert should be passed in the 4th parameter. For the first table joined
   *   on a query, this value is ignored as the first table is taken as the base
   *   table. The token %alias can be used in this string to be replaced with
   *   the actual alias. This is useful when $alias is modified by the database
   *   system, for example, when joining the same table more than once.
   * @param $arguments
   *   An array of arguments to replace into the $condition of this join.
   * @return
   *   The unique alias that was assigned for this table.
   */
  public function leftJoin($table, $alias = NULL, $condition = NULL, $arguments = array());

  /**
   * Right Outer Join against another table in the database.
   *
   * @param $table
   *   The table against which to join.
   * @param $alias
   *   The alias for the table. In most cases this should be the first letter
   *   of the table, or the first letter of each "word" in the table.
   * @param $condition
   *   The condition on which to join this table. If the join requires values,
   *   this clause should use a named placeholder and the value or values to
   *   insert should be passed in the 4th parameter. For the first table joined
   *   on a query, this value is ignored as the first table is taken as the base
   *   table. The token %alias can be used in this string to be replaced with
   *   the actual alias. This is useful when $alias is modified by the database
   *   system, for example, when joining the same table more than once.
   * @param $arguments
   *   An array of arguments to replace into the $condition of this join.
   * @return
   *   The unique alias that was assigned for this table.
   */
  public function rightJoin($table, $alias = NULL, $condition = NULL, $arguments = array());

  /**
   * Join against another table in the database.
   *
   * This method does the "hard" work of queuing up a table to be joined against.
   * In some cases, that may include dipping into the Schema API to find the necessary
   * fields on which to join.
   *
   * @param $type
   *   The type of join. Typically one one of INNER, LEFT OUTER, and RIGHT OUTER.
   * @param $table
   *   The table against which to join. May be a string or another SelectQuery
   *   object. If a query object is passed, it will be used as a subselect.
   * @param $alias
   *   The alias for the table. In most cases this should be the first letter
   *   of the table, or the first letter of each "word" in the table. If omitted,
   *   one will be dynamically generated.
   * @param $condition
   *   The condition on which to join this table. If the join requires values,
   *   this clause should use a named placeholder and the value or values to
   *   insert should be passed in the 4th parameter. For the first table joined
   *   on a query, this value is ignored as the first table is taken as the base
   *   table. The token %alias can be used in this string to be replaced with
   *   the actual alias. This is useful when $alias is modified by the database
   *   system, for example, when joining the same table more than once.
   * @param $arguments
   *   An array of arguments to replace into the $condition of this join.
   * @return
   *   The unique alias that was assigned for this table.
   */
  public function addJoin($type, $table, $alias = NULL, $condition = NULL, $arguments = array());

  /**
   * Orders the result set by a given field.
   *
   * If called multiple times, the query will order by each specified field in the
   * order this method is called.
   *
   * If the query uses DISTINCT or GROUP BY conditions, fields or expressions
   * that are used for the order must be selected to be compatible with some
   * databases like PostgreSQL. The PostgreSQL driver can handle simple cases
   * automatically but it is suggested to explicitly specify them. Additionally,
   * when ordering on an alias, the alias must be added before orderBy() is
   * called.
   *
   * @param $field
   *   The field on which to order.
   * @param $direction
   *   The direction to sort. Legal values are "ASC" and "DESC".
   * @return SelectQueryInterface
   *   The called object.
   */
  public function orderBy($field, $direction = 'ASC');

  /**
   * Orders the result set by a random value.
   *
   * This may be stacked with other orderBy() calls. If so, the query will order
   * by each specified field, including this one, in the order called. Although
   * this method may be called multiple times on the same query, doing so
   * is not particularly useful.
   *
   * Note: The method used by most drivers may not scale to very large result
   * sets. If you need to work with extremely large data sets, you may create
   * your own database driver by subclassing off of an existing driver and
   * implementing your own randomization mechanism. See
   *
   * http://jan.kneschke.de/projects/mysql/order-by-rand/
   *
   * for an example of such an alternate sorting mechanism.
   *
   * @return SelectQueryInterface
   *   The called object
   */
  public function orderRandom();

  /**
   * Restricts a query to a given range in the result set.
   *
   * If this method is called with no parameters, will remove any range
   * directives that have been set.
   *
   * @param $start
   *   The first record from the result set to return. If NULL, removes any
   *   range directives that are set.
   * @param $length
   *   The number of records to return from the result set.
   * @return SelectQueryInterface
   *   The called object.
   */
  public function range($start = NULL, $length = NULL);

  /**
   * Add another Select query to UNION to this one.
   *
   * Union queries consist of two or more queries whose
   * results are effectively concatenated together. Queries
   * will be UNIONed in the order they are specified, with
   * this object's query coming first. Duplicate columns will
   * be discarded. All forms of UNION are supported, using
   * the second '$type' argument.
   *
   * Note: All queries UNIONed together must have the same
   * field structure, in the same order. It is up to the
   * caller to ensure that they match properly. If they do
   * not, an SQL syntax error will result.
   *
   * @param $query
   *   The query to UNION to this query.
   * @param $type
   *   The type of UNION to add to the query. Defaults to plain
   *   UNION.
   * @return SelectQueryInterface
   *   The called object.
   */
  public function union(SelectQueryInterface $query, $type = '');

  /**
   * Groups the result set by the specified field.
   *
   * @param $field
   *   The field on which to group. This should be the field as aliased.
   * @return SelectQueryInterface
   *   The called object.
   */
  public function groupBy($field);

  /**
   * Get the equivalent COUNT query of this query as a new query object.
   *
   * @return SelectQueryInterface
   *   A new SelectQuery object with no fields or expressions besides COUNT(*).
   */
  public function countQuery();

  /**
   * Indicates if preExecute() has already been called on that object.
   *
   * @return
   *   TRUE is this query has already been prepared, FALSE otherwise.
   */
  public function isPrepared();

  /**
   * Generic preparation and validation for a SELECT query.
   *
   * @return
   *   TRUE if the validation was successful, FALSE if not.
   */
  public function preExecute(SelectQueryInterface $query = NULL);

  /**
   * Helper function to build most common HAVING conditional clauses.
   *
   * This method can take a variable number of parameters. If called with two
   * parameters, they are taken as $field and $value with $operator having a value
   * of IN if $value is an array and = otherwise.
   *
   * @param $field
   *   The name of the field to check. If you would like to add a more complex
   *   condition involving operators or functions, use having().
   * @param $value
   *   The value to test the field against. In most cases, this is a scalar. For more
   *   complex options, it is an array. The meaning of each element in the array is
   *   dependent on the $operator.
   * @param $operator
   *   The comparison operator, such as =, <, or >=. It also accepts more complex
   *   options such as IN, LIKE, or BETWEEN. Defaults to IN if $value is an array
   *   = otherwise.
   * @return QueryConditionInterface
   *   The called object.
   */
  public function havingCondition($field, $value = NULL, $operator = NULL);

  /**
   * Clone magic method.
   *
   * Select queries have dependent objects that must be deep-cloned.  The
   * connection object itself, however, should not be cloned as that would
   * duplicate the connection itself.
   */
  public function __clone();

  /**
   * Add FOR UPDATE to the query.
   *
   * FOR UPDATE prevents the rows retrieved by the SELECT statement from being
   * modified or deleted by other transactions until the current transaction
   * ends. Other transactions that attempt UPDATE, DELETE, or SELECT FOR UPDATE
   * of these rows will be blocked until the current transaction ends.
   *
   * @param $set
   *   IF TRUE, FOR UPDATE will be added to the query, if FALSE then it won't.
   *
   * @return QueryConditionInterface
   *   The called object.
   */
  public function forUpdate($set = TRUE);
}

/**
 * The base extender class for Select queries.
 */
class SelectQueryExtender implements SelectQueryInterface {

  /**
   * The SelectQuery object we are extending/decorating.
   *
   * @var SelectQueryInterface
   */
  protected $query;

  /**
   * The connection object on which to run this query.
   *
   * @var DatabaseConnection
   */
  protected $connection;

  /**
   * A unique identifier for this query object.
   */
  protected $uniqueIdentifier;

  /**
   * The placeholder counter.
   */
  protected $placeholder = 0;

  public function __construct(SelectQueryInterface $query, DatabaseConnection $connection) {
    $this->uniqueIdentifier = uniqid('', TRUE);
    $this->query = $query;
    $this->connection = $connection;
  }

  /**
   * Implements QueryPlaceholderInterface::uniqueIdentifier().
   */
  public function uniqueIdentifier() {
    return $this->uniqueIdentifier;
  }

  /**
   * Implements QueryPlaceholderInterface::nextPlaceholder().
   */
  public function nextPlaceholder() {
    return $this->placeholder++;
  }

  /* Implementations of QueryAlterableInterface. */

  public function addTag($tag) {
    $this->query->addTag($tag);
    return $this;
  }

  public function hasTag($tag) {
    return $this->query->hasTag($tag);
  }

  public function hasAllTags() {
    return call_user_func_array(array($this->query, 'hasAllTags'), func_get_args());
  }

  public function hasAnyTag() {
    return call_user_func_array(array($this->query, 'hasAnyTags'), func_get_args());
  }

  public function addMetaData($key, $object) {
    $this->query->addMetaData($key, $object);
    return $this;
  }

  public function getMetaData($key) {
    return $this->query->getMetaData($key);
  }

  /* Implementations of QueryConditionInterface for the WHERE clause. */

  public function condition($field, $value = NULL, $operator = NULL) {
    $this->query->condition($field, $value, $operator);
    return $this;
  }

  public function &conditions() {
    return $this->query->conditions();
  }

  public function arguments() {
    return $this->query->arguments();
  }

  public function where($snippet, $args = array()) {
    $this->query->where($snippet, $args);
    return $this;
  }

  public function compile(DatabaseConnection $connection, QueryPlaceholderInterface $queryPlaceholder) {
    return $this->query->compile($connection, $queryPlaceholder);
  }

  public function compiled() {
    return $this->query->compiled();
  }

  /* Implementations of QueryConditionInterface for the HAVING clause. */

  public function havingCondition($field, $value = NULL, $operator = '=') {
    $this->query->condition($field, $value, $operator, $num_args);
    return $this;
  }

  public function &havingConditions() {
    return $this->having->conditions();
  }

  public function havingArguments() {
    return $this->having->arguments();
  }

  public function having($snippet, $args = array()) {
    $this->query->having($snippet, $args);
    return $this;
  }

  public function havingCompile(DatabaseConnection $connection) {
    return $this->query->havingCompile($connection);
  }

  /* Implementations of QueryExtendableInterface. */

  public function extend($extender_name) {
    // The extender can be anywhere so this needs to go to the registry, which
    // is surely loaded by now.
    $class = $this->connection->getDriverClass($extender_name, array(), TRUE);
    return new $class($this, $this->connection);
  }

  /* Alter accessors to expose the query data to alter hooks. */

  public function &getFields() {
    return $this->query->getFields();
  }

  public function &getExpressions() {
    return $this->query->getExpressions();
  }

  public function &getOrderBy() {
    return $this->query->getOrderBy();
  }

  public function &getGroupBy() {
    return $this->query->getGroupBy();
  }

  public function &getTables() {
    return $this->query->getTables();
  }

  public function &getUnion() {
    return $this->query->getUnion();
  }

  public function getArguments(QueryPlaceholderInterface $queryPlaceholder = NULL) {
    return $this->query->getArguments($queryPlaceholder);
  }

  public function isPrepared() {
    return $this->query->isPrepared();
  }

  public function preExecute(SelectQueryInterface $query = NULL) {
    // If no query object is passed in, use $this.
    if (!isset($query)) {
      $query = $this;
    }

    return $this->query->preExecute($query);
  }

  public function execute() {
    // By calling preExecute() here, we force it to preprocess the extender
    // object rather than just the base query object.  That means
    // hook_query_alter() gets access to the extended object.
    if (!$this->preExecute($this)) {
      return NULL;
    }

    return $this->query->execute();
  }

  public function distinct($distinct = TRUE) {
    $this->query->distinct($distinct);
    return $this;
  }

  public function addField($table_alias, $field, $alias = NULL) {
    return $this->query->addField($table_alias, $field, $alias);
  }

  public function fields($table_alias, array $fields = array()) {
    $this->query->fields($table_alias, $fields);
    return $this;
  }

  public function addExpression($expression, $alias = NULL, $arguments = array()) {
    return $this->query->addExpression($expression, $alias, $arguments);
  }

  public function join($table, $alias = NULL, $condition = NULL, $arguments = array()) {
    return $this->query->join($table, $alias, $condition, $arguments);
  }

  public function innerJoin($table, $alias = NULL, $condition = NULL, $arguments = array()) {
    return $this->query->innerJoin($table, $alias, $condition, $arguments);
  }

  public function leftJoin($table, $alias = NULL, $condition = NULL, $arguments = array()) {
    return $this->query->leftJoin($table, $alias, $condition, $arguments);
  }

  public function rightJoin($table, $alias = NULL, $condition = NULL, $arguments = array()) {
    return $this->query->rightJoin($table, $alias, $condition, $arguments);
  }

  public function addJoin($type, $table, $alias = NULL, $condition = NULL, $arguments = array()) {
    return $this->query->addJoin($type, $table, $alias, $condition, $arguments);
  }

  public function orderBy($field, $direction = 'ASC') {
    $this->query->orderBy($field, $direction);
    return $this;
  }

  public function orderRandom() {
    $this->query->orderRandom();
    return $this;
  }

  public function range($start = NULL, $length = NULL) {
    $this->query->range($start, $length);
    return $this;
  }

  public function union(SelectQueryInterface $query, $type = '') {
    $this->query->union($query, $type);
    return $this;
  }

  public function groupBy($field) {
    $this->query->groupBy($field);
    return $this;
  }

  public function forUpdate($set = TRUE) {
    $this->query->forUpdate($set);
    return $this;
  }

  public function countQuery() {
    // Create our new query object that we will mutate into a count query.
    $count = clone($this);

    // Zero-out existing fields and expressions.
    $fields =& $count->getFields();
    $fields = array();
    $expressions =& $count->getExpressions();
    $expressions = array();

    // Also remove 'all_fields' statements, which are expanded into tablename.*
    // when the query is executed.
    $tables = &$count->getTables();
    foreach ($tables as $alias => &$table) {
      unset($table['all_fields']);
    }

    // Ordering a count query is a waste of cycles, and breaks on some
    // databases anyway.
    $orders = &$count->getOrderBy();
    $orders = array();

    // COUNT() is an expression, so we add that back in.
    $count->addExpression('COUNT(*)');

    return $count;
  }

  function isNull($field) {
    $this->query->isNull($field);
    return $this;
  }

  function isNotNull($field) {
    $this->query->isNotNull($field);
    return $this;
  }

  public function exists(SelectQueryInterface $select) {
    $this->query->exists($select);
    return $this;
  }

  public function notExists(SelectQueryInterface $select) {
    $this->query->notExists($select);
    return $this;
  }
  
  public function __toString() {
    return (string) $this->query;
  }

  public function __clone() {
    $this->uniqueIdentifier = uniqid('', TRUE);

    // We need to deep-clone the query we're wrapping, which in turn may
    // deep-clone other objects.  Exciting!
    $this->query = clone($this->query);
  }

  /**
   * Magic override for undefined methods.
   *
   * If one extender extends another extender, then methods in the inner extender
   * will not be exposed on the outer extender.  That's because we cannot know
   * in advance what those methods will be, so we cannot provide wrapping
   * implementations as we do above.  Instead, we use this slower catch-all method
   * to handle any additional methods.
   */
  public function __call($method, $args) {
    $return = call_user_func_array(array($this->query, $method), $args);

    // Some methods will return the called object as part of a fluent interface.
    // Others will return some useful value.  If it's a value, then the caller
    // probably wants that value.  If it's the called object, then we instead
    // return this object.  That way we don't "lose" an extender layer when
    // chaining methods together.
    if ($return instanceof SelectQueryInterface) {
      return $this;
    }
    else {
      return $return;
    }
  }
}

/**
 * Query builder for SELECT statements.
 */
class SelectQuery extends Query implements SelectQueryInterface {

  /**
   * 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 SelectQuery 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();

  /**
   * The conditional object for the WHERE clause.
   *
   * @var DatabaseCondition
   */
  protected $where;

  /**
   * The conditional object for the HAVING clause.
   *
   * @var DatabaseCondition
   */
  protected $having;

  /**
   * Whether or not this query should be DISTINCT
   *
   * @var boolean
   */
  protected $distinct = FALSE;

  /**
   * The range limiters for this query.
   *
   * @var array
   */
  protected $range;

  /**
   * An array whose elements specify a query to UNION, and the UNION type. The
   * 'type' key may be '', 'ALL', or 'DISTINCT' to represent a 'UNION',
   * 'UNION ALL', or 'UNION DISTINCT' statement, respectively.
   *
   * All entries in this array will be applied from front to back, with the
   * first query to union on the right of the original query, the second union
   * to the right of the first, etc.
   *
   * @var array
   */
  protected $union = array();

  /**
   * Indicates if preExecute() has already been called.
   * @var boolean
   */
  protected $prepared = FALSE;

  /**
   * The FOR UPDATE status
   */
  protected $forUpdate = FALSE;

  public function __construct($table, $alias = NULL, DatabaseConnection $connection, $options = array()) {
    $options['return'] = Database::RETURN_STATEMENT;
    parent::__construct($connection, $options);
    $this->where = new DatabaseCondition('AND');
    $this->having = new DatabaseCondition('AND');
    $this->addJoin(NULL, $table, $alias);
  }

  /* Implementations of QueryAlterableInterface. */

  public function addTag($tag) {
    $this->alterTags[$tag] = 1;
    return $this;