Oracle rownum和order by一起使用时对结果集的影响

Oracle版本：V9.0

当单独使用order by对数据集进行排序时，SQL如下：

select gend_name,PC_COST03,PC_COST02,pc_start,(case when gend_name is null then PC_COST03 else PC_COST02 end) as pc_cost 
from pc_mstr
left join gend_det on gend_gen = 'PC_MSTR' and pc_part like Trim(gend_option) || '%' and gend_disabled=0
where 1=1 
and pc_vend = '715R1736' 
and pc_part = '7001-32L82F-0U7A1C11'
--and rownum = 1
order by pc_start desc

结果：

　

当rownum和order by一起使用时，SQL入下：

select gend_name,PC_COST03,PC_COST02,pc_start,(case when gend_name is null then PC_COST03 else PC_COST02 end) as pc_cost 
from pc_mstr
left join gend_det on gend_gen = 'PC_MSTR' and pc_part like Trim(gend_option) || '%' and gend_disabled=0
where 1=1 
and pc_vend = '715R1736' 
and pc_part = '7001-32L82F-0U7A1C11'
and rownum = 1
order by pc_start desc

结果：

原本以为逻辑没问题，值应该是130，但执行结果却是113，经反复推敲，最后得出的结果是，Oracle在执行此语句时，先给了一个记录给rownum，然后才进行order by，最后才会导致这个结果。因此，考虑到程序在多行数据集中取值时，若无指定取的是某一行，则默认取第一行的值，因此最后的解决办法是，放弃使用rownum，由程序默认取值进行处理。

最终SQL如下：

select gend_name,PC_COST03,PC_COST02,pc_start,(case when gend_name is null then PC_COST03 else PC_COST02 end) as pc_cost 
from pc_mstr
left join gend_det on gend_gen = 'PC_MSTR' and pc_part like Trim(gend_option) || '%' and gend_disabled=0
where 1=1 
and pc_vend = '715R1736' 
and pc_part = '7001-32L82F-0U7A1C11'
order by pc_start desc