code: private Column add(String schema, String table, String column, int stepDepth, String alias) {
          Column c = new Column(schema, table, column, this.filteredAllTables.get(schema + "." + table).get(column), stepDepth);
          this.columns.put(c, alias);
          this.aliases.put(alias, c);
          return c;
      }
docs: add a new column
  
  @param schema
  @param table
  @param column
  @param stepDepth
  @param alias