private function change_column_null($table_name, $column_name, $null, $default = null)
{
if ($null === false || $default !== null) {
$sql = sprintf("UPDATE %s SET %s=%s WHERE %s IS NULL", $this->quote_table_name($table_name), $this->quote_column_name($column_name), $this->quote($default), $this->quote_column_name($column_name));
$this->query($sql);
}
$sql = sprintf("ALTER TABLE %s ALTER %s %s NOT NULL", $this->quote_table_name($table_name), $this->quote_column_name($column_name), $null ? 'DROP' : 'SET');
$this->query($sql);
}