sql - How to count over several relations in SQL

Question

I have a table structure, which, simplified, looks something like this.

grandparents
  id
  name
parents
  id
  grandparent_id
  name
children
  id
  parent_id
  name

I know, I can count the number of children a parent has like this:

select
  name,
  ( select count (*) from children c where c.parent_id = p.id ) as count
  from p parents;

My question is, how do I count the number of children that are related to a grandparent. The table structure cannot be changed, and I want a single SELECT statement. Is that possible?

Thanks in advance.

Answer 1

This should do the trick, even in weird family trees (parents being brothers for example):

SELECT gp.id,
       gp.name,
       COUNT(DISTINCT c.id)
FROM   grandparents gp
       INNER JOIN parents p
               ON p.grandparent_id = gp.id
       INNER JOIN children c
               ON c.parent_id = p.id
GROUP  BY gp.id,
          gp.name  

See fiddle (no Firebird engine available, but this code is SQL standard compliant).