为什么在使用dbms_random.value函数时需要在此Oracle SQL查询中包含rownum字段？

I am pulling a random sample from some tables and noticed that depending on how I write the query it doesn't work. Obviously, I wasn't using all_tab_columns, I was just providing an example that would work on a vanilla instance (9.2.0.8).I am pulling a random sample from some tables a