Where is the error in this query? (sql-ex.ru exercise 25)

Question

I'm trying to solve Exercise 25 from sql-ex.ru:

The database scheme consists of four tables:

• Product(maker, model, type)
• PC(code, model, speed, ram, hd, cd, price)
• Laptop(code, model, speed, ram, hd, screen, price)
• Printer(code, model, color, type, price)

Find the printer makers also producing PCs with the lowest RAM capacity and the highest processor speed of all PCs having the lowest RAM capacity. Result set: maker.

I can't see why the following query does not produce the desired result on the second checking database:

SELECT DISTINCT product.maker
FROM product
RIGHT JOIN printer ON printer.model = product.model
WHERE product.maker IN(
    SELECT product.maker
    FROM product
    RIGHT JOIN pc ON pc.model = product.model
    WHERE pc.ram = (SELECT MIN(ram) FROM pc)
    AND pc.speed = (
        SELECT MAX(t.speed)
        FROM (SELECT speed FROM pc WHERE ram = (SELECT MIN(ram) FROM pc)) AS t
    )
)

The error is as follows:

Wrong
Your query produced correct result set on main database, but it failed test on second, checking database
* Wrong number of records (less by 1)

Why does it fail to pick exactly 1 maker?

Answer 1

I do not know where is the error in your query, but you can use something like this:

WITH t AS ( SELECT * FROM PC c WHERE c.ram = (SELECT MIN(ram) FROM PC) ) SELECT DISTINCT a.maker FROM Product a WHERE a.type = 'PC' AND a.maker IN ( SELECT b.maker FROM Product b WHERE b.type = 'Printer' ) AND a.model IN ( SELECT model FROM t WHERE speed = (SELECT MAX(speed) FROM t) ) 

Answer 2

Here's the solution to this problem:

WITH CTE_Product AS
(
    SELECT DISTINCT
        Maker
    FROM
        Product
    WHERE
        Type = 'PC'
    AND
        Maker IN (SELECT Maker FROM Product WHERE Type = 'Printer')
)

select c.maker from CTE_Product C
inner join
(
select distinct maker from product inner join PC on product.model=pc.model 
where type='PC'and  
pc.speed in 
          (select max(speed) 
           from pc 
           where ram=
                     (select min(ram)from pc)
           )
 and pc.ram=
             (select min(ram)
              from pc)
) A on A.maker = C.maker

Answer 3

SELECT DISTINCT maker
  FROM Product
 WHERE TYPE = 'Printer'

 INTERSECT

SELECT DISTINCT maker
  FROM Product AS p
    INNER JOIN PC as pc
    ON p.model = pc.model
    WHERE
      type = 'PC'
      AND (pc.ram = ALL (SELECT MIN(ram) FROM PC))
        AND (pc.speed = (SELECT MAX(speed) FROM PC WHERE ram = (SELECT MIN(ram) FROM PC )))