code: protected List<SelectItem> getSelectItems(PlainSelect plainSelect) {
          //设置selectItems
          List<SelectItem> selectItems = new ArrayList<SelectItem>();
          for (SelectItem selectItem : plainSelect.getSelectItems()) {
              //别名需要特殊处理
              if (selectItem instanceof SelectExpressionItem) {
                  SelectExpressionItem selectExpressionItem = (SelectExpressionItem) selectItem;
                  if (selectExpressionItem.getAlias() != null) {
                      //直接使用别名
                      Column column = new Column(selectExpressionItem.getAlias().getName());
                      SelectExpressionItem expressionItem = new SelectExpressionItem(column);
                      selectItems.add(expressionItem);
                  } else if (selectExpressionItem.getExpression() instanceof Column) {
                      Column column = (Column) selectExpressionItem.getExpression();
                      SelectExpressionItem item = null;
                      if (column.getTable() != null) {
                          Column newColumn = new Column(column.getColumnName());
                          item = new SelectExpressionItem(newColumn);
                          selectItems.add(item);
                      } else {
                          selectItems.add(selectItem);
                      }
                  } else {
                      selectItems.add(selectItem);
                  }
              } else if (selectItem instanceof AllTableColumns) {
                  selectItems.add(new AllColumns());
              } else {
                  selectItems.add(selectItem);
              }
          }
          // SELECT *, 1 AS alias FROM TEST
          // 应该为
          // SELECT * FROM (SELECT *, 1 AS alias FROM TEST)
          // 不应该为
          // SELECT *, alias FROM (SELECT *, 1 AS alias FROM TEST)
          for (SelectItem selectItem : selectItems) {
              if (selectItem instanceof AllColumns) {
                  return Collections.singletonList(selectItem);
              }
          }
          return selectItems;
      }
docs: 获取查询列
  
  @param plainSelect
  @return