Converts a query result to a boolean. If the result is a database error according to $this->isError(), this returns false; otherwise, this returns true.
This method should be overridden by subclasses. This method is called by the constructor to set values in $this->sql, which is an array keyed on sql name.
This method returns an Association object from storage that matches the server URL and, if specified, handle. It returns null if no such association is found or if the matching association is expired.