code: @Override
      //CHECKSTYLE:OFF
      public DerbyDatabase addTableColumn(final Connection _con,
                                          final String _tableName,
                                          final String _columnName,
                                          final ColumnType _columnType,
                                          final String _defaultValue,
                                          final int _length,
                                          final int _scale)
          throws SQLException
      {
        //CHECKSTYLE:ON
          String defaultValue = _defaultValue;
  
          if (defaultValue == null)  {
              switch (_columnType)  {
                  case INTEGER:
                  case REAL:
                      defaultValue = "0";
                      break;
                  case DATETIME:
                  case STRING_LONG:
                  case STRING_SHORT:
                      defaultValue = "''";
                      break;
                  default:
                      break;
              }
          }
          return super.addTableColumn(_con, _tableName, _columnName, _columnType,
                                      defaultValue, _length, _scale);
      }
docs: Adds a column to a SQL table. The method overrides the original method,
  because Derby does not allow for <code>NOT NULL</code> columns that no
  default value is defined. Is such column is created, the default value
  for real and integer is <code>0</code>, for date time, short and long
  string a zero length string.
  
  @param _con          SQL connection
  @param _tableName    name of table to update
  @param _columnName   column to add
  @param _columnType   type of column to add
  @param _defaultValue default value of the column (or null if not
  specified)
  @param _length       length of column to add (or 0 if not specified)
  @param _scale        scale of the column to add (or 0 if not
  specified)
  @throws SQLException if the column could not be added to the tables
  @return this