How Does (GROUP BY)'S Compilation Work ??

Hi,
I'm trying to know how the procedure done when using (group by) and how the result will be ordered without idintical data note even with these data the result still ambiguous , for example :
if we have a table has 2 columns (empno,ename)

empno    1    2    3    . ...........  26     
ename    a    b    c    . ............ z

then execute --------------------->

select ename , empno
from test
group by ename , empno

the result has to be to retrieve the table without changes but what happened illogical , so please help me to know the logic of (group by) .