    # File lib/active_record/connection_adapters/mysql_adapter.rb, line 486
          def change_column_null(table_name, column_name, null, default = nil)
            column = column_for(table_name, column_name)

            unless null || default.nil?
              execute("UPDATE #{quote_table_name(table_name)} SET #{quote_column_name(column_name)}=#{quote(default)} WHERE #{quote_column_name(column_name)} IS NULL")
            end

            change_column table_name, column_name, column.sql_type, :null => null
          end