code: protected void addMapping(final ColumnType _columnType,
                                final String _writeTypeName,
                                final String _nullValueSelect,
                                final String... _readTypeNames)
      {
          this.writeColTypeMap.put(_columnType, _writeTypeName);
          this.nullValueColTypeMap.put(_columnType, _nullValueSelect);
          for (final String readTypeName : _readTypeNames) {
              Set<AbstractDatabase.ColumnType> colTypes = this.readColTypeMap.get(readTypeName);
              if (colTypes == null) {
                  colTypes = new HashSet<>();
                  this.readColTypeMap.put(readTypeName, colTypes);
              }
              colTypes.add(_columnType);
          }
      }
docs: Adds a new mapping for given eFaps column type used for mapping from and
  to the SQL database.
  
  @param _columnType column type within eFaps
  @param _writeTypeName SQL type name used to write (create new column
  within a SQL table)
  @param _nullValueSelect null value select used within the query if a link
  target could be a null (and so all selected values must null
  in the SQL statement for objects without this link)
  @param _readTypeNames list of SQL type names returned from the database
  meta data reading
  @see #readColTypeMap to map from an eFaps column type to a SQL type name
  @see #writeColTypeMap to map from a SQL type name to possible eFaps
  column types