Sql-ex, what are the features of the "Computer Firm" test database?

Question

PostgreSQL

Good afternoon, I'm solving problems from the sql-ex.ru website and for several days now I've been puzzling over the 75th problem, it seems that the problem is not difficult, but the 2nd check base says otherwise)

Initially, I joined each table with the Product table

select 
  maker,
  max(l.price) as laptop,
  max(pc.price) as pc,
  max(r.price) as printer
from 
  Product t
  left join Laptop l 
    on t.model=l.model
  left join PC 
    on t.model=pc.model
  left join Printer r 
    on t.model=r.model
group by 
  maker

Did not accept

After, decided to merge tables

with
  B as (
    select 
      p.maker, p.type, max(a.price) 
    from 
      Product p 
      inner join (
        select distinct model, price from Laptop
        union
        select distinct model, price from PC
        union
        select distinct model, price from Printer
      ) a 
      on 
        p.model=a.model 
    group by 
      p.maker, p.type
    )

select 
  b1.maker,
  coalesce((select max from B where maker=b1.maker and type='Laptop'), null) as Laptop,
  coalesce((select max from B where maker=b1.maker and type='PC'), null) as PC,
  coalesce((select max from B where maker=b1.maker and type='Printer'), null) as Printer
from 
  B b1
group by 
  b1.maker

What was also rejected by the verification database)

The question is, what did I forget to take into account, what is the feature of the verification database "Computer Firm"?

Answer 1

You forgot the condition "... which have products with a known price in at least one of the tables..."
That is, you need to:

select 
  maker,
  max(l.price) as laptop,
  max(pc.price) as pc,
  max(r.price) as printer
from 
  Product t
  left join Laptop l 
    on t.model=l.model
  left join PC 
    on t.model=pc.model
  left join Printer r 
    on t.model=r.model
group by 
  maker
having
  max(l.price) IS NOT NULL
  OR max(pc.price) IS NOT NULL
  OR max(r.price) IS NOT NULL