Question

I have this SQL query which calculates total weight:

select c.componentid, nvl(cs.weightkg, 0) as componentkg,
 (case 
    when exists (select 1 from component where fkcomponentid = c.componentid) then
      (select sum(nvl(cs.weightkg, 0)) as kg FROM  component a, componentstats cs  where a.fkcomponentid is not null and cs.componentstatsid = a.componentstatsid and a.fkcomponentid = c.componentid)
 end) as childrenkg
 from component c, componentstats cs  
 where 
 cs.componentstatsid = c.componentstatsid
 and componentid = ?
 order by c.componentid;

How I can rewrite it into hierarchical query? Is this possible?

EDIT: Picture of the scheme

Answer 1

The basic query would be like this:

select the_level, c.componentid, NVL( cs.weightkg,0) as componentkg
  from ( select level the_level, componentid from component
           start with componentid = ?
           connect by fkcomponentid = prior componentid
       ) c
  join componentstats using (componentstatsid)
order by the_level, c.componentid

That would give you each individual component and its weight. You could then group and sum in various ways — either by level if you want the sum of the subcomponents’ weights as in your original query, or over everything if you just want a total weight.