def column_type
      return @column_type if defined?(@column_type)
      ind = self.index
      table[1..500].each do |row|
        if row[ind] and row[ind].cell_type
          cel_column_type = row[ind].cell_type
          if !defined?(@column_type) or @column_type.nil?
            @column_type = cel_column_type
          elsif cel_column_type == @column_type or cel_column_type == :nil
          else
            @column_type = :string
            break
          end
        end
      end
      return @column_type
    end
def column_type
      return @column_type if defined?(@column_type)
      ind = self.index
      table[1..500].each do |row|
        if row[ind] and row[ind].cell_type
          cel_column_type = row[ind].cell_type
          if !defined?(@column_type) or @column_type.nil?
            @column_type = cel_column_type
          elsif cel_column_type == @column_type or cel_column_type == :nil
          else
            @column_type = :string
            break
          end
        end
      end
      return @column_type
    end
character limit
 Returns column type, either :primary_key, :string, :text, :integer, :float, :decimal, :datetime, :date, :binary, :boolean