oracle rownum

rownum背后的机制： 
1 Oracle executes your query.

2 Oracle fetches the first row and calls it row number 1.

3 Have we gotten past row number meets the criteria? If no, then Oracle discards the row, If yes, then Oracle return the row.

4 Oracle fetches the next row and advances the row number (to 2, and then to 3, and then to 4, and so forth).

5 Go to step 3.

简单翻译：

1.先按照where中除了rownum以外的其他的条件查询出结果并生成结果集 
2.给第一步生成的结果集加上rownum伪列 
3.按照rownum的条件进一步限制。如果一条记录不符合rownum的限制条件，则丢弃，而且下一条记录的rownum还是从1开始重新计数 

 

oracle的 rownum机制 这篇文章讲解的很清楚了！