Is it possible to make a recursive SQL query?

Question

I have a table similar to this:

CREATE TABLE example (
  id integer primary key,
  name char(200),
  parentid integer,
  value integer);

I can use the parentid field to arrange data into a tree structure.

Now here's the bit I can't work out. Given a parentid, is it possible to write an SQL statement to add up all the value fields under that parentid and recurse down the branch of the tree ?

UPDATE: I'm using posgreSQL so the fancy MS-SQL features are not available to me. In any case, I'd like this to be treated as a generic SQL question.

Answer 1

There are a few ways to do what you need in PostgreSQL.

• If you can install modules, look at the tablefunc contrib. It has a connectby() function that handles traversing trees. http://www.postgresql.org/docs/8.3/interactive/tablefunc.html

• Also check out the ltree contrib, which you could adapt your table to use: http://www.postgresql.org/docs/8.3/interactive/ltree.html

• Or you can traverse the tree yourself with a PL/PGSQL function.

Something like this:

create or replace function example_subtree (integer)
returns setof example as
'declare results record;
         child record;
 begin
  select into results * from example where parent_id = $1;
  if found then
    return next results;
    for child in select id from example
                  where parent_id = $1
      loop
        for temp in select * from example_subtree(child.id)
        loop
          return next temp;
        end loop;
      end loop;
  end if;
  return null;
end;' language 'plpgsql';

select sum(value) as value_sum
  from example_subtree(1234);