interface QueryConditionInterface {

  /**
   * Helper function: builds the most common 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.
   *
   * Do not use this method to test for NULL values. Instead, use
   * QueryConditionInterface::isNull() or QueryConditionInterface::isNotNull().
   *
   * @param $field
   *   The name of the field to check. If you would like to add a more complex
   *   condition involving operators or functions, use where().
   * @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 $value is
   *   an array, and = otherwise.
   *
   * @return QueryConditionInterface
   *   The called object.
   *
   * @see QueryConditionInterface::isNull()
   * @see QueryConditionInterface::isNotNull()
   */
  public function condition($field, $value = NULL, $operator = NULL);

  /**
   * Adds an arbitrary WHERE clause to the query.
   *
   * @param $snippet
   *   A portion of a WHERE clause as a prepared statement. It must use named
   *   placeholders, not ? placeholders.
   * @param $args
   *   An associative array of arguments.
   *
   * @return QueryConditionInterface
   *   The called object.
   */
  public function where($snippet, $args = array());

  /**
   * Sets a condition that the specified field be NULL.
   *
   * @param $field
   *   The name of the field to check.
   *
   * @return QueryConditionInterface
   *   The called object.
   */
  public function isNull($field);

  /**
   * Sets a condition that the specified field be NOT NULL.
   *
   * @param $field
   *   The name of the field to check.
   *
   * @return QueryConditionInterface
   *   The called object.
   */
  public function isNotNull($field);

  /**
   * Sets a condition that the specified subquery returns values.
   *
   * @param SelectQueryInterface $select
   *   The subquery that must contain results.
   *
   * @return QueryConditionInterface
   *   The called object.
   */
  public function exists(SelectQueryInterface $select);

  /**
   * Sets a condition that the specified subquery returns no values.
   *
   * @param SelectQueryInterface $select
   *   The subquery that must not contain results.
   *
   * @return QueryConditionInterface
   *   The called object.
   */
  public function notExists(SelectQueryInterface $select);

  /**
   * Gets a complete list of all conditions in this conditional clause.
   *
   * This method returns by reference. That allows alter hooks to access the
   * data structure directly and manipulate it before it gets compiled.
   *
   * The data structure that is returned is an indexed array of entries, where
   * each entry looks like the following:
   * @code
   * array(
   *   'field' => $field,
   *   'value' => $value,
   *   'operator' => $operator,
   * );
   * @endcode
   *
   * In the special case that $operator is NULL, the $field is taken as a raw
   * SQL snippet (possibly containing a function) and $value is an associative
   * array of placeholders for the snippet.
   *
   * There will also be a single array entry of #conjunction, which is the
   * conjunction that will be applied to the array, such as AND.
   */
  public function &conditions();

  /**
   * Gets a complete list of all values to insert into the prepared statement.
   *
   * @return
   *   An associative array of placeholders and values.
   */
  public function arguments();

  /**
   * Compiles the saved conditions for later retrieval.
   *
   * This method does not return anything, but simply prepares data to be
   * retrieved via __toString() and arguments().
   *
   * @param $connection
   *   The database connection for which to compile the conditionals.
   * @param $queryPlaceholder
   *   The query this condition belongs to. If not given, the current query is
   *   used.
   */
  public function compile(DatabaseConnection $connection, QueryPlaceholderInterface $queryPlaceholder);

  /**
   * Check whether a condition has been previously compiled.
   *
   * @return
   *   TRUE if the condition has been previously compiled.
   */
  public function compiled();
}