Sql-ex what are the main features testing database "Computer firm"?

Good afternoon math site sql-ex.ru and for several days scratching their heads over 75 puzzles, like the puzzle is not difficult, but 2 test base says otherwise)

Initially I jainil 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

Not accepted

After, I decided to combine 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

Which was also rejected by the test base)

The question that I forgot to consider what a feature of the test database "Computer firm"?
April 7th 20 at 15:52
1 answer
April 7th 20 at 15:54
Solution
You forgot the condition "... that have a product with a known price of at least one of the tables..."

That is necessary:
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

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