How to retrieve related data which has a composite primary key and corresponding foreign key in the other multiple tables?

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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).

This was first published in September 2005

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