[Err] ORA-00923: FROM keyword not found where expected 与row

[Err] ORA-00923: FROM keyword not found where expected 与rownum，ora-00923rownum

关于oracle的nownum

如果我想查询表的全部信息,并且前面加上行号,sql如下

select  ROWNUM,* from tableA

会报下面的错误

[Err] ORA-00923: FROM keyword not found where expected

得给表加上别名

SELECT  ROWNUM,a.* ROWNUM FROM tableA   a

为什么需要这样,我还没深入学习,大家谁知道,请告诉我