Is it possible to do a 'Select FIRST x - records' in SQL

I'm having a production order file wich contains Po-nbr and a week

Po-nbr      : nprod

Start-week : dplsw

Tool-nbr   : ntool

 

 

I want to retrieve  only the first PO-nbr to start.... but it is possible to
have 2 or more Po's starting the same date... in that case I only want the
first PO-nbr

 

What I did so far ...

with whpo (wntool, wdplsw)                              

 as (select ntool, min(dplsw)  from pofile

     group by ntool)                                  

select nprod, dplsw, ntool, wntool, wdplsw        

from pofile left outer join whpo                 

   on wntool=ntool and wdplsw=dplsw                 

 

But the what I want to avoid 'duplicate records... what I would need in fact
is a select first....

 

Anyone good ideas ? 

 

 

Thierry