code: public static Column var(DataRowsFacade dataFrame, String columnName) {
          return dataFrame.get().groupBy(columnName).agg(functions.variance(columnName)).col(columnName);
      }
docs: Standard deviation for a column
  
  @param dataFrame  the dataframe to
  get the column from
  @param columnName the name of the column to get the standard
  deviation for
  @return the column that represents the standard deviation