code: public final void parse(final SelectStatement selectStatement) {
          if (!lexerEngine.skipIfEqual(DefaultKeyword.GROUP)) {
              return;
          }
          lexerEngine.accept(DefaultKeyword.BY);
          while (true) {
              addGroupByItem(basicExpressionParser.parse(selectStatement), selectStatement);
              if (!lexerEngine.equalAny(Symbol.COMMA)) {
                  break;
              }
              lexerEngine.nextToken();
          }
          lexerEngine.skipAll(getSkippedKeywordAfterGroupBy());
          selectStatement.setGroupByLastIndex(lexerEngine.getCurrentToken().getEndPosition() - lexerEngine.getCurrentToken().getLiterals().length() - 1);
      }
docs: Parse group by.
  
  @param selectStatement select statement