docs: private List<From> findFrom(SQLTableSource from) {
          //zhongshu-comment class1.isAssignableFrom(class2) class2是不是class1的子类或者子接口
          //改成用instanceof 应该也行吧：from instanceof SQLExprTableSource
          boolean isSqlExprTable = from.getClass().isAssignableFrom(SQLExprTableSource.class);
  
          if (isSqlExprTable) {
              SQLExprTableSource fromExpr = (SQLExprTableSource) from;
              String[] split = fromExpr.getExpr().toString().split(",");
  
              ArrayList<From> fromList = new ArrayList<>();
              for (String source : split) {
                  fromList.add(new From(source.trim(), fromExpr.getAlias()));
              }
              return fromList;
          }
  
          SQLJoinTableSource joinTableSource = ((SQLJoinTableSource) from);
          List<From> fromList = new ArrayList<>();
          fromList.addAll(findFrom(joinTableSource.getLeft()));
          fromList.addAll(findFrom(joinTableSource.getRight()));
          return fromList;
      }