Retrieve data related by composite key

As you suggested, related tables are related through their primary and foreign keys. In order to retrieve related data, we simply join the tables on all the columns in their keys:

select dept.DVID as dept_division
     , dept.DPID as dept_id
     , sect.SCID as section_id
     , sect.SCNAME as section_name
     , count(EMPNO) as staff_count
  from DEPARTMENT as dept
inner
  join SECTION as sect
    on dept.DVID = sect.DVID
   and dept.DPID = sect.DPID
left outer
  join EMPLOYEE as empl
    on sect.DVID = empl.DVID
   and sect.DPID = empl.DPID     
   and sect.SCID = empl.SCID
group
    by dept.DVID
     , dept.DPID
     , sect.SCID
     , sect.SCNAME

For additional examples involving these tables, see Composite foreign keys (13 September 2002).